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Resonant light-matter interactions in nanophotonic structures: for manipulating optical forces and thermal emission
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Resonant light-matter interactions in nanophotonic structures: for manipulating optical forces and thermal emission
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i
Resonant Light-Matter Interactions in Nanophotonic
Structures: For Manipulating Optical Forces and Thermal
Emission
by
Aravind Krishnan
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the Requirements for the Degree
DOCTOR OF PHILOSOPHY
(Department of Electrical and Computer Engineering)
December 2019
ii
I would like to dedicate this thesis to my dear wife Indu Aravind for her unconditional love and
support throughout my graduate school.
iii
Acknowledgements
I am extremely grateful to my research advisor Prof. Michelle Povinelli for her support and
mentoring all these years. I consider myself very fortunate to find such a positive and
encouraging person as my guide. I would like to thank her for the freedom she had given me
on my projects. She dedicated significant amount of her time discussing the results and editing
my manuscripts. It was such a joy working with her over these years and I would like to thank
her from the depth of my heart.
I would like to sincerely thank all members and alumni’s of Dr.Povinelli’s Nanophotonics lab.
Dr.Ningfeng Huang was my mentor when I joined the lab. He helped me learn the
nanofabrication tools, device characterization techniques and optical trapping methods. Along
with Ningfeng, I have to thank Dr.Roshni Biswas and Dr.Duke Anderson for their advices,
training on fabrication and measurement tools and most importantly for their warm friendship.
I am extremely thankful to my colleagues Dr.Shao-Hua Wu, Ahmed Morsy and Romil
Audhkhasi. They were the best lab mates I could hope for. Excellent researchers and great
friends. We had collaborated and helped each other out on several projects. I with them all the
success in their life and research career.
I am also grateful to the faculty of USC Viterbi School, especially, Prof. Stephen Cronin, Prof.
Jayakanth Ravichandran, Prof. Wei Wu, Prof. Rehan Kapadia, and Prof. Hossein Hashemi for
their help and considerations. The staff of Viterbi School were extremely supportive over these
years. Angela Woertler and Kim Reid does an excellent job in handling finances and help with
iv
the purchases. I would also like to thank Dr.Donghai Zhu and Alfonso Jimenez for their efforts
in maintaining the fabrication facilities.
I am deeply indebted to my family and friends for their incredible support. I can’t thank enough
my loving wife Indu Aravind for suffering me through my Ph.D. days. Without her
unconditional support, it wouldn’t have been possible to finish my dissertation. I also owe a
great debt of gratitude to my parents Ramakrishnan Nair and Sujatha Kumari and my dear
brother Anandakrishnan. They have always been there when I needed them.
Finally, I would like to thank my favorite soccer teams FC Barcelona and Liverpool FC for
bringing joy to my weekends during the busy school years.
v
Abstract
This dissertation work studies the use of resonant light interactions in nanophotonic structures
to control the flow of light in two major application areas: Manipulating optical forces and
thermal emission.
When light interact with matter having length scales comparable to the wavelength, the
interactions result in interesting phenomena which are otherwise inaccessible. The spatio-
spectral distribution of light can be modified using engineered materials with the help of
resonant interactions. In this dissertation I am probing into the use of two such resonant
phenomena in Nanophotonic structures. Guided resonance in dielectric nanostructures in Part
I with the emphasize in controlling the optical forces and surface plasmon resonance in metallic
nanostructures in Part II for modifying thermal emissions
The Guided resonance in photonic crystals has been extensively used for guiding the light in
integrated photonics. The spatial confinement of light resulting from the resonance can be used
to generate powerful optical force to manipulate microscopic matter. First, I had experimentally
demonstrated the optical trapping of submicron objects various sizer and composition using
highly enhanced optical traps in photonic crystals. Applicability of optical trapping tools for
nanomanipulation is limited by the available laser power and trap efficiency. The strong
confinement of light in a slot-graphite photonic crystal is utilized to develop high-efficiency
parallel trapping over a large area. The stiffness is 35 times higher than our previously
vi
demonstrated on-chip, near field traps. We demonstrate the ability to trap both dielectric and
metallic particles of sub-micron size.
The strong field enhancement leads to secondary light matter interactions via optical heating.
We harness residual thermal effects in a low-absorptivity system to manipulate parallel optical
trapping of particles on the nanoscale. We show that the size selectivity of the trap can be tuned
by adding a non-ionic surfactant to the solution, altering the thermophoretic effect that delivers
nanoparticles to trapping sites. We find that the growth kinetics of nanoparticle arrays on the
slot-graphite template depends on particle size. This difference is exploited to selectively trap
one type of particle out of a binary colloidal mixture, creating an efficient optical sieve. This
technique has rich potential for analysis, diagnostics, and enrichment and sorting of
microscopic entities. We further show that particles can be permanently immobilized on the
photonic crystal via photopolymerization of the trapping medium.
Optical assembly has potential applications in bottom up fabrication of nanostructured
materials. The ability to dynamically manipulate the configuration of optical assemblies can
lead to reconfigurable photonic matter with variable optical properties. We present a photonic-
crystal design which supports multiple guided-resonance modes in a narrow spectral range.
Introduction of mutually-orthogonal slots within a conventional lattice allows us to create
polarization-sensitive guided modes with distinct near-field periodicities and tunable resonance
wavelengths. The device can potentially be used as a reconfigurable optical trap, multiband
tunable filter.
We have also explored the macroscopic applications of optical forces in photonic crystals. The
Breakthrough Starshot initiative has proposed to use laser radiation pressure to propel a
vii
lightsail to an exoplanet. One major challenge is the effect of laser beam distortion on sail
stability. We propose and investigate the use of lightsails based on Kerr nonlinear photonic
crystals as a passive method for increasing sail stability. The key concept is to flatten the
dependence of reflected power on incident power at the laser wavelength, using a specially
designed, guided-resonance mode of the nonlinear photonic crystal. We use coupled-mode
theory to analyze the resonance characteristics that yield the flattest curve. We then design a
silicon nitride photonic crystal that supports a resonance with the desired properties. We show
that our design simultaneously provides both high stability and high thrust on the sail, unlike
designs based on linear materialsr, or differential sensor.
In Part II, the surface plasmon resonance in metallic nanostructures is explored for modifying
the thermal emission in engineered nanostructures. Nanophotonic structures can be used to
break the fundamental constrains on conventional thermal emitters and realize narrow spectral
response. We present a switchable system with narrowband emission using metallic gratings
and phase change materials. The phase change material turns a surface plasmon mode on or off
by its insulator to metal phase transition. We proposed two design architectures and analyze
the limits on the emission switching within the frame work of coupled mode theory.
viii
Table of Contents
List of Figures ....................................................................................................................... x
List of Tables ..................................................................................................................... xiv
Part I: Optical Forces in Nanophotonic structures .................................................................. 1
Chapter 1 Introduction ..................................................................................................... 2
1.1 Optical force ........................................................................................................... 2
1.2 Gradient forces for micromanipulation .................................................................... 3
1.3 Guided Resonance Modes in Photonic Crystals ....................................................... 6
1.4 Radiation pressure and optical propulsion ............................................................... 7
Chapter 2 Nearfield optical trapping in slot-photonic crystals ........................................... 9
2.1 Introduction ............................................................................................................ 9
2.2 Device design ....................................................................................................... 11
2.3 Fabrication and Characterization ........................................................................... 14
2.4 Experiment ........................................................................................................... 16
2.5 Summary .............................................................................................................. 21
Chapter 3 Manipulating secondary light interactions: Opto-Thermophoretic Assembly .. 22
3.1 Introduction .......................................................................................................... 22
3.2 Optical absorption in the trapping system .............................................................. 24
3.3 Experiment ........................................................................................................... 25
3.4 Permanent Optical Assembly ................................................................................ 32
3.5 Conclusion ............................................................................................................ 34
Chapter 4 Spatio-Spectral light management: Reconfigurable Optical Assembly ............ 36
4.1 Introduction .......................................................................................................... 36
4.2 Dual-slot photonic crystal design .......................................................................... 37
4.3 Fabrication and characterization ............................................................................ 44
4.4 Conclusion ............................................................................................................ 47
Chapter 5 Stabilizing Optical Forces: Laser Sail ............................................................. 49
5.1 Introduction .......................................................................................................... 49
5.2 Effect of Nonlinear Resonance on Stability and Thrust.......................................... 52
5.3 Design Implementation of Light Sail ..................................................................... 58
5.4 Conclusion ............................................................................................................ 62
ix
Chapter 6 Conclusions and Outlook ............................................................................... 64
6.1 Conclusion ............................................................................................................ 64
6.2 Outlook ................................................................................................................. 66
Part II Thermal emission in Nanophotonic structures.......................................................... 67
Chapter 7 Introduction ................................................................................................... 68
7.1 Tailoring thermal emission .................................................................................... 68
7.2 Surface Plasmons in nanostructures ....................................................................... 70
7.3 VO2 phase change material ................................................................................... 71
Chapter 8 Thermally switchable narrowband emission using phase change materials ..... 74
8.1 Introduction .......................................................................................................... 74
8.2 Theory .................................................................................................................. 75
8.3 Results and Discussion .......................................................................................... 78
8.4 Conclusion ............................................................................................................ 88
Chapter 9 Conclusions and Outlook ............................................................................... 89
9.1 Conclusion ............................................................................................................ 89
9.2 Outlook ................................................................................................................. 89
References .......................................................................................................................... 95
x
List of Figures
Figure 2.1 Schematic view of optical trapping using a slot-graphite photonic crystal. Incident
light from below excites a guided-resonance mode of the photonic-crystal slab, giving rise to
optical forces on nanoparticles in colloidal solution ............................................................. 11
Figure 2.2 a) Graphite photonic crystal template b) Slot-graphite photonic crystal template 11
Figure 2.3 Slot graphite photonic crystal design. a) One unit-cell of the slot-graphite photonic
crystal. b) Transmission spectrum of the device. (c) Intensity enhancement at resonance for y-
polarized incident light. d) In-plane force distribution on top of the photonic crystal ........... 14
Figure 2.4 Fabrication and characterization. a) SEM image of the Slot-graphite photonic crystal
device used in the experiment. The scale bar represents 1 µm. (c) Measured transmission
spectrum of the device. ........................................................................................................ 15
Figure 2.5 Trap stiffness for incident y-polarized light. (a) Histogram of stiffness values in the
direction perpendicular to the polarization of the incident light. (b) Stiffness in the direction
parallel to the incident polarization ...................................................................................... 17
Figure 2.6 Trapping in slot-graphite lattice. Assembly of (a) 520nm polystyrene particles (b)
300nm gold nanoparticles.................................................................................................... 18
Figure 2.7 Instantaneous number of trapped particles for a colloidal solution containing equal
concentrations of (a) 380nm or 520nm polystyrene alone. (b) 380nm and 520nm polystyrene
particles together. ................................................................................................................ 20
Figure 3.1 Schematic of optical trap based on photonic crystal. An array of optical traps is
created upon normal incidence illumination by a laser. ........................................................ 23
Figure 3.2 Absorbed power in the unit cell of the photonic crystal. a) Coordinate system with
origin at the center of the slot, x axis along slot width and y axis along slot height. Power
absorption along b) x-y plane c) y-z plane d) x-z plane of the slot for an incident intensity of
1.75μW/ μm
2
. ...................................................................................................................... 25
Figure 3.3 Particle density in the trapping region for (a) small, 380nm-diameter particles and
(b) large, 520nm-diameter particles. The grey area represents the time during which trapping
laser is on. The inset shows the spatial distribution of particles in the active region before
turning off the beam at 1200s; each green circle labels a particle position............................ 27
Figure 3.4 Temporal evolution of optical trapping from a binary mixture of 380nm and 520nm
particles. Spatial distribution of the assembly from the mixture is shown in the inset. Small
particles are shown in green circles and large ones in red .................................................... 28
Figure 3.5 Particle density in the trapping area for (a) small 380nm particles and (b) large
520nm particles with Triton-X. The grey area represents the time at which trapping laser is on.
Spatial distribution of particles in the active region before turning off the beam at 1200 s is
shown in the inset ................................................................................................................ 30
xi
Figure 3.6 Temporal evolution of optical trapping from a binary mixture of 380nm and 520nm
particles with Triton-X. Spatial distribution of the assembly from the mixture is shown in the
inset. Small particles are shown in green circles and large ones in red ................................. 31
Figure 3.7 Tuning the size selectivity of the optical traps by changing the surfactant
concentration....................................................................................................................... 32
Figure 3.8 Optical trapping and permanent immobilization. (a) Before Trapping. (b) Trapping
laser is turned on and trapping is completed. Trapped particles are exposed to UV light with
trapping laser on. (c) Polymerized matrix. Particles stay immobilized in the matrix even after
the trapping laser is turned off. (d) Fluorescent image of green 380 particles in the matrix. (e)
Fluorescent image of red 520 particles in the matrix ............................................................ 34
Figure 4.1 a,b) In-plane electric field vector (arrows) and out-of-plane magnetic field
component (color map) at the center of the photonic-crystal slab formed by a square lattice of
holes in silicon for (a) x- and (b) y-polarized incident light, prior to the introduction of slots. c)
Locations of center slots (red) and edge slots (green) introduced into the square lattice. d) The
design parameters of the dual-slot photonic crystal. a and r are the lattice constant and hole
radius of the original square lattice. The width and height of the center slots are represented by
wc, hc, and the edge slots by we, he. ...................................................................................... 39
Figure 4.2 a) The transmission spectrum of the device for different slot heights. b-d) The
magnetic (Hz) and electric field (|E|
2
) distribution at the center of the square lattice of holes for
(b, d) x- and (c, e) y- polarized light. f-i) The magnetic (Hz) and electric field (|E|
2
) distribution
at the center of the dual-slot photonic crystal for (f, h) x- and (g, i) y- polarized light. .......... 40
Figure 4.3 a) The resonant wavelength as a function of r and a for x- and y- polarizations. b)
The difference in resonant wavelengths for y- polarized and x- polarized modes as a function
of r and a. ........................................................................................................................... 41
Figure 4.4 The resonance wavelengths for x- and y- polarizations with a) width and b) height
of edge slots, c) width and d) height of center slots. ............................................................. 42
Figure 4.5 The quality factor of resonances for x- and y polarizations with a) width and b)
height of edge slots, c) width and d) height of center slots. .................................................. 43
Figure 4.6 SEM image of one of the devices. The scale bar represents 1µm. ....................... 45
Figure 4.7 Transmission spectrum of dual-slot devices for a) different lattice constants and b)
different hole radius. ........................................................................................................... 46
Figure 4.8 Quality factor of the fabricated devices as a function of the a) lattice constant and
b) hole radii. Circles denote the measured Q; stars denote the value of Q from simulation. .. 47
Figure 5.1 a) Intensity profile of incident laser beam at the sail surface: ideal (red) and
perturbed (blue). b) Reflected power as a function of incident power from a linear sail and a
nonlinear sail. c) Radiation pressure distribution across the sail. An asymmetric distribution
results in unwanted torque. .................................................................................................. 51
Figure 5.2 a) Intensity profile of incident laser beam at the sail surface: ideal (solid) and actual
(dashed). b) Reflected power as a function of incident power from linear material and nonlinear
photonic crystal. c) Radiation pressure distribution across the sail. An asymmetric distribution
results in unwanted torque. .................................................................................................. 54
xii
Figure 5.3 a) Average slope of the input-reflected power curve. b) Average reflected power.
Regions of parameter space where the curve is multivalued are excluded from the plot
(indicated by grey region). .................................................................................................. 57
Figure 5.4 a) Average slope of the input-reflected power curve. b) Average reflected power.
Regions of parameter space where the curve is multivalued are excluded from the plot
(indicated by grey region). .................................................................................................. 58
Figure 5.5 a) Schematic of silicon nitride photonic crystal slab consisting of a square lattice of
air holes of radius 0.22a, where a = 1µm is the lattice constant, and a thickness of 0.76a. b)
Reflection spectrum of the structure. c) Dependence of reflected power on incident power. 59
Figure 5.6 a) Average slope of the reflected power vs. input power curve and b) average
reflected power for different detuning. c) Reflected power response of linear materials and
nonlinear photonic crystal. .................................................................................................. 61
Figure 7.1 a) The crystal structure of VO2 below and above phase transition temperature. The
refractive index of VO2 when the material is in b) insulating and c) metallic state ............... 72
Figure 8.1 Mapping resonances to Γ𝑟𝑎𝑑 − Γ𝑎𝑏𝑠 parameter space. Resonance A has a quality
factor of 100 and peak absorption of 1. Resonance B has a quality factor of 100 and peak
absorption of 0.19. Resonance C has a quality factor of 1000 and peak absorption of 1 ....... 78
Figure 8.2 Aluminum grating on glass substrate. a) Schematic. b) Electric field intensity (|E|
2
)
of grating with w = 1µm, h = 0.4µm, t = 1µm, and a = 10µm. c) Absorption of Aluminum-air
grating with varying periodicity for normally incident light polarized perpendicular to the
grating................................................................................................................................. 80
Figure 8.3 a) Aluminum grating on glass substrate with VO2-filled grooves. The grating
structure with b) insulating and c) metallic VO2. Absorptivity of grating with d) insulating and
e) metallic VO2 for varying lattice constants. Structure dimensions: w = 1 µm, h = 0.4 µm, t =
1 µm. f-g) Electric field intensity distribution corresponding to resonance at 10µm for f)
insulating and g) metallic state of VO2 ................................................................................ 81
Figure 8.4 Peak absorption (background color map) and quality factor (dashed contour lines)
as a function of the absorptive and radiative decay rates, for various choices of grating
parameters w and h for the grating geometry of Fig. 8.2(a). ................................................. 83
Figure 8.5 a) Aluminum-ZnS grating on glass substrate with a VO2 top layer. b) Electric field
intensity (|E|
2
) in the insulating state of VO2. c) Electric field intensity (|E|
2
) in the metallic
state of VO2. d) Absorption of grating with insulating VO2 for varying lattice constants. e)
Absorptivity of grating with metallic VO2 for varying lattice constants. Structure dimensions:
w = 1 µm, h = 0.4 µm, t = 1 µm, tf = 100nm. ....................................................................... 85
Figure 8.6 a) Aluminum-ZnS grating on Silica substrate with VO2 shunts. b,c) Absorption of
grating with b) insulating and c) metallic VO2 for varying shunt width (a = 10µm, w = 1µm, h
= 0.4µm, t = 1µm, ts = 100nm). d,e) ) Absorption of grating with b) insulating and c) metallic
VO2 for varying shunt thickness (a = 10µm, w = 1µm, h = 0.4µm, t = 1µm, ws = 1.5w). ...... 86
xiii
Figure 8.7 Peak absorption (background color map) and quality factor (dashed contour lines)
as a function of the absorptive and radiative decay rates, with varying shunt thickness(ts) and
width (ws) for a grating periodicity a = 10µm. The red curve is for a VO2 shunt covers only the
grating gaps and blue curve is for a continuous VO2 film .................................................... 88
Figure 9.1 Remote heating: Problem and Proposed Solution. a) the schematic of the device
illuminated with laser beam. b) Intensities of ideal and distorted Gaussian beam across the
device (shaded grey region). The range of power fluctuations is shown in shaded orange. c)
Absorbed power vs incident power for linear (black) and nonlinear (green) device. d)
Temperature profile across the linear device (blue and red) and nonlinear device (green) .... 92
Figure 9.2 a) The schematic of the optical thermoregulatory. b) The shift in resonance with
feedback signal c) The switching between states and temperature oscillations. d) Pinning down
of temperature in the device ................................................................................................ 93
xiv
List of Tables
Table 8.1. Wavelength and quality factor of absorption peaks for aluminum grating ........... 80
1
Part I:
Optical Forces in Nanophotonic structures
2
Introduction
1.1 Optical force
Light can exert force on matter by means of momentum exchange [1, 2]. These forces can
either push the objects away in the propagation direction of the light beam (Radiation pressure
or Scattering force) or attract the objects towards high intensity regions in a non-uniform beam
intensity distribution (Gradient force) [3]. Optical forces are widely utilized in microscopic
regime for immobilizing [4, 5], sorting [6-8], and transporting matter [9, 10]. These forces can
have immense application in many scientific areas, including optics [11, 12], atomic physics
[2, 13-15], biological science [16-19] and chemistry
[20-22].
While the tools based on optical gradient forces continue to unravel the mysteries of the
microscopic world, the scattering forces can help us to conquer length scales unachievable with
our current technologies. Radiation pressure from an unfocused beam of light can be used to
propel objects in space [23-25]. In the wake of alarming rate of climatic changes and depletion
of earthly resources, it is of great need in ascertaining the characteristics of the environment
prevailing outside our solar system. The NASA Starlight [26] and Breakthrough Starshot
initiative [27] have proposed to use laser radiation pressure to propel a lightsail to an exoplanet
[28, 29]. Solar propelled sails have been successfully built and launched by the Japanese
3
Aerospace Exploration Agency (JAXA) with the IKAROS spacecraft,[30] by NASA with the
NanoSail-D spacecraft [31], and by The Planetary Society with the LightSail 1 spacecraft [32].
As optical forces are being used in a plethora of applications ranging from micromanipulation
to interplanetary missions, this section of the thesis explores the role of resonant nanophotonic
structures in pushing the limits of these forces. In part I of this thesis, generation and
manipulation of optical forces in photonic crystals structures via guided resonance modes is
presented. Chapter 2 to 4 deals with the application of optical forces in the microscopic regime,
In Chapter 2, it will be shown that a slotted photonic crystal lattice can be used for versatile
optical assemblies with enhanced trapping performance. In Chapter 3, the secondary light-
matter interactions are utilized to manipulate the optical forces to achieve tunable assemblies
of nanoparticles. An advanced photonic crystal design is presented in Chapter 4 to achieve
dynamically reconfigurable assembly of nanomaterials. Chapter 5 presents the macroscopic
use of optical forces. Combining with the nonlinear effects, the guided resonance in photonic
crystals can be used for stabilizing the perturbations in radiation pressure. A photonic crystal
design is presented which has the capability to be used as a sail for laser propelled mission to
Alpha Centauri star systems.
1.2 Gradient forces for micromanipulation
The field of Optical manipulation has experienced intensive development ever since the
discovery of optical tweezers which uses forces exerted by a strongly focused beam of light to
trap small objects [4, 33]. Optical forces ranging from femtonewtons to nanonewtons are now
4
widely being used to immobilize, transport and sort mesoscopic systems with characteristic
length scales ranging from tens of nanometres to hundreds of micrometres [2].
Small objects develop an electric dipole moment in response to the light's electric field. The
resulting optical forces can be regarded as the sum of forces exerted on the induced dipoles and
the interaction between the dipoles [2]. The time-averaged electromagnetic force acting on a
dipole is given by:[1]
2 * * 0
1
Re( ) | | Re( )
4 2 4
c
F E E H E E
ci
= + + (1.1)
where α is the dipolar polarizability, σ is the extinction cross-section, E the electric field, H the
magnetic field, c the speed of light in vacuum, and ω the angular frequency of the optical field.
The first term in equation (1.1) is the force due to the gradient of the electric field intensity.
The second term corresponds to radiation pressure. The third term is a force arising from the
presence of spatial polarization gradients. If the gradient force dominates over other terms, the
light can confine the object in a stable potential [1].
When the size of the object is much larger compared to the wavelength of light, the dipolar
approximation doesn’t hold well. In this regime, the full electromagnetic scattering theory must
be employed. Larger objects refract the light rays and redirect the momentum of the incoming
photons. The resulting recoil draws them toward the higher flux of photons near the focus [2].
The rate of change of momentum is equal in magnitude and opposite in sign to the time-
averaged radiation force acting on the center of mass of the particle [1]. When a particle cannot
be approximated as a dipole, Frad can be calculated by integrating the optical momentum flux
over a closed orientable surface S surrounding the object [3].
5
.
rad M
S
F T dS =
(1.2)
where TM is the Maxwell stress tensor, accounting for the interaction between electromagnetic
forces and mechanical momentum, which can be calculated from the scattered fields, and dS is
an outward-directed element of surface area. The Maxwell stress tensor method can be used to
calculate the forces on particles of any size and shape in response to an electromagnetic field.
Using conventional optical tweezers, it is difficult to stably and accurately trap particles much
smaller than the wavelength of light. The main problem for trapping nanoparticles is that the
gradient force decreases rapidly with decreasing particle size (the force is proportional to the
third power of the particle radius). Small particles may easily escape the trapping potential well
due to Brownian motion [3]. The conventional optical tweezers are diffraction limited which
places a lower bound on size to which light can be focused [34]. The minimum spot size
diameter is given by
min
1.2 / d NA = , where NA is the numerical aperture of the lens and λ is
the trapping wavelength. Since light intensity is given by the input power divided by the
illuminated area, this hinders the accurate trap confinement and places a fundamental constrain
on the trapping nanoparticle using diffraction limited tweezers [35]. Simply using higher
numerical aperture lenses or increasing the laser power does not significantly improve the
optical tweezer performance. In addition, high-power lasers cause instant damage to the
particles due to thermal effects [3].
Fortunately, optical nanotweezers based on nearfield of integrated photonic structures offer an
alternative approach to scale the trapped objects down to the nanoscale. In the case where there
6
exists a significant gradient in the intensity of the light field the polarization induced force takes
the form: [35]
0
2
grad
I
F
c
= (1.3)
where
0
I is the gradient in intensity in the nearfield and is the polarizability of the particle.
When the intensity gradient and polarizability are of same sign, the gradient force attracts the
particles towards the region of higher intensity.
1.3 Guided Resonance Modes in Photonic Crystals
Photonic crystal slabs, featuring two dimensional periodic index contrast introduced into a
high-index guiding layer [36, 37], can be used to generate gradient forces for optical
manipulation [38-41]. The existence of standing Bloch modes, also known as guided resonance
modes, in these structures can produce resonantly enhanced optical near-field. The slab
structure supports in-plane guided modes which are completely confined to the slab [42]. These
modes cannot be excited with external sources due to the conservation of in-plane wave vector
[43]. The introduction of periodic index perturbation, via hole introduction, reduces the
translational symmetry of the slab [43]. The in-plane wave vector is conserved only up to the
reciprocal lattice vector, thus allowing incident light to couple to guided modes of the slab. The
guided modes which couple to radiation modes possess a finite lifetime but still retain
significant portions of the electromagnetic power within the dielectric slab [43]. The guided
resonance modes interfere with the Fabry Perot oscillations of the slab giving rise to Fano
shaped resonances on a smoothly varying background [43, 44]. The guided modes exist at the
7
Γ-points in the brillouin zone can be excited by a normally incident plane wave with appropriate
symmetry properties [44]. Most of the silicon-based photonic crystal slabs focus on guided
resonance modes in the optical communication band, where silicon is non-absorptive, resulting
in very high quality (Q) factor modes [45].
1.4 Radiation pressure and optical propulsion
Light carries momentum with momentum density given by
2
/ P S c = in free space, where S is
the energy flux and c is the speed of light [46]. The linear momentum carried by the
electromagnetic wave changes, when light gets absorbed or change its direction upon
interaction with matter. This change in momentum translates as a reactive force which tends to
push the objects in the direction of propagation of light. On the macroscopic scale, Maxwell-
Bartoli force expression can be used to account for the force that light exerts onto a surface.
Optical force can be expressed by energy per unit distance or momentum per unit time. The
Radiation pressure P is the rate of change of momentum divided by the unit area given by: [47]
2
(1 ) cos
I
PR
c
=+ (1.4)
where I is the irradiance (the energy arriving at the surface per unit time (input power) per unit
area), and θ is the angle between the surface normal and the incident radiation and R is the
reflectivity of the surface.
The radiation pressure on macroscopic objects can be made much higher than its weight by
engineering the structures to have high reflectivity and provide sufficiently high beam intensity.
Objects can be levitated and accelerated in this manner by using high-power laser beams [23-
8
25, 27]. This can find huge applications in next generation lightweight satellites or propulsion
of cosmic light sails [24]. This type of propulsion eliminates the need of on-board fuel on the
spacecraft and could enable ultra-fast, even relativistic space-flight speeds necessary for
scalable space exploration.
Using optical propulsion for interstellar travel requires lightweight, highly reflective and
mechanically compliant mirrors [23]. In this aspect, photonic crystal membranes supporting
guided resonance are ideal alternatives to conventional mirrors [48], as they provide high
reflectivity with only a single suspended layer of patterned dielectric material. Photonic
crystals made of low loss materials can realize mirrors with sub-wavelength thicknesses and
reflectivity > 99 %, mostly limited by scattering losses. Low-pressure chemically vapor-
deposited silicon nitride (LPCVD SiN) is one such example at NIR wavelengths [49]. They
can also high intrinsic stress, and weak coupling to undesired thermal modes [48].
9
Nearfield optical trapping in slot-photonic crystals
A version of the results in this chapter was published as Ref. [40]
2.1 Introduction
Optical trapping serves as a powerful tool for the manipulation of matter on the nanoscale [1,
2, 50, 51]. The ability to immobilize multiple nano objects on a substrate will play a key role
in future integrated analytical platforms [1, 52, 53]. At a given laser power, the applicability of
trapping techniques is limited by the trap efficiency [54], often quantified by trap stiffness.
Developing highly efficient and versatile optical trapping systems will facilitate new
experiments and applications in broad areas from physics to biology [55-58].
To enhance the trapping performance, several past approaches have engineered the trapped
objects [54, 59, 60] or taken advantage of their special properties, such as plasmonic resonances
[61-63]. However, such approaches limit the application range. A more flexible approach is to
improve the efficiency of the trap itself. The optical near fields of plasmonic and integrated
photonic structures provide strong optical gradients, resulting in highly efficient traps [10, 35,
64-72]. Among these, all-dielectric designs provide absorption-free operation with negligible
heating.
10
In our earlier work, we realized an array of near-field traps using the structured light fields
above a 2D silicon photonic crystal [69]. In order to further enhance the trapping performance,
we theoretically proposed to include slots within each unit cell of the photonic crystal lattice
[73]. Due to the boundary conditions on the electric field, the narrow slot in the dielectric
strongly enhances the electric field intensity [74], creating a highly efficient optical trap.
Previously, we have fabricated such a device, known as a slot-graphite photonic crystal, and
characterized its optical mode [75].
Here, we experimentally demonstrate the optical trapping of nanoparticles in our design. The
trap stiffness is 35 times higher than our previously demonstrated photonic crystal traps [69],
based on simple square lattice. We demonstrate trapping of dielectric as well as metallic
nanoparticles of different sizes using the same photonic crystal device. We also study the
growth kinetics of nanoparticle clusters in the photonic crystal trap. The difference in kinetics
with respect to the particle size is used to selectively trap one type of particles out of a mixture,
creating an efficient optical sieve. We expect this capability to be of widespread interest for
microfluidic, lab-on-chip applications. A schematic view of optical trapping in the slot-graphite
template is shown in Fig. 2.1.
11
Figure 2.1 Schematic view of optical trapping using a slot-graphite photonic crystal. Incident light from
below excites a guided-resonance mode of the photonic-crystal slab, giving rise to optical forces on
nanoparticles in colloidal solution
2.2 Device design
The slot-graphite device is designed by introducing narrow rectangular slots in each unit cell
of a conventional graphite lattice. The template is made of silicon, and the pattern of holes and
slots is arranged in a modified graphite lattice [75] as shown in Fig. 2.2.
Figure 2.2 a) Graphite photonic crystal template b) Slot-graphite photonic crystal template
12
The slots create high index contrast interfaces across which the normal component of electric
flux density(D) has to be continuous according to the Maxwell’s equations. As a result, the
electric field (E-field) must undergo a large discontinuity with much higher amplitude in the
low-index side of the interface [74]. The field intensity remains high all over the slot if the
width of the slot is much smaller than the characteristic decay length inside the slot.
The slot-graphite photonic crystal is designed to support a guided resonance mode around
1550nm, where the silicon absorption is negligible. When the incident light wavelength is tuned
to excite the mode, the optical field is confined within the slot and the local electromagnetic
field intensity is enhanced [73]. The electric-field gradient just above the slab surface attracts
the nanoparticles towards the traps
Figure 2.3(a) shows one unit-cell of the slot-graphite lattice. The photonic crystal has a lattice
constant (a) of 820nm, air hole radius (r/a) of 0.155, slot width (wx) of 550nm, and slot height
(wy) of 90nm. Figure 2.3(b) and 2.3(c) show the transmission spectrum of the device and the
resonant enhancement of electric field intensity (|E|
2
) at the center of the photonic crystal slab
(z=0 plane) respectively. Numerical simulations are carried out using the 3D finite-difference
time domain (FDTD) method (Lumerical) with y-polarized incident light. The optical force on
a nanoparticle due to the field gradient in the photonic crystal can be calculated by using the
Maxwell’s stress tensor as:
.
ij
S
F T ndS =
(2.1)
13
where i and j are one of the three primary coordinate directions (x, y or z), S is an arbitrary
surface around the object, n is the unit normal of S, and Tij is the Maxwell Stress Tensor [76],
which has the following form:
* * 2 2
1
( | | | | )
2
i j i j ij
Tij E E H H E H = + − + (2.2)
The force distribution on a 100nm particle 25nm above the slab is shown in Fig. 2.3(d).
Particles are expected to trap in the high intensity slot region.
14
Figure 2.3 Slot graphite photonic crystal design. a) One unit-cell of the slot-graphite photonic crystal. b)
Transmission spectrum of the device. (c) Intensity enhancement at resonance for y-polarized incident light. d)
In-plane force distribution on top of the photonic crystal
2.3 Fabrication and Characterization
We fabricated the slot-graphite photonic crystal on a silicon-on-insulator wafer (SOITEC) with
a 250nm thick silicon layer on top of a 3µm silica layer [69, 75]. The backside of the wafer
was polished to a mirror finish with an average surface roughness less than 10 nm. A 220nm
layer of Si3N4 was deposited on the backside using PECVD to reduce the reflection. The sample
was spin-coated with PMMA-A4 950K. A 50μm diameter slot-graphite pattern was exposed
using Raith 150 e-beam system with an acceleration voltage of 30 kV and beam aperture of
15
10µm. The pattern was transferred from the resist to the silicon layer by ICP-RIE etching using
a modified Bosch process with gas mixture of SF 6 and C4F8. The SEM image of the photonic
crystal device is shown in Fig. 2.4(a).
Figure 2.4 Fabrication and characterization. a) SEM image of the Slot-graphite photonic crystal device used
in the experiment. The scale bar represents 1 µm. (c) Measured transmission spectrum of the device.
The transmission spectrum of the fabricated device was characterized in cross polarization
mode at normal incidence [77] and is shown in Fig. 2.4(b). The guided resonance mode is
strongly confined to the slab and appears as a peak in the transmission spectrum [77]. The
resonant wavelength and quality factor are determined to be 1559nm and 1217, respectively,
by fitting the experimental spectrum to a Fano function. A Santec TSL-550 tunable laser (1500-
1620nm) was used for the optical characterization. An aspherical lens (f = 11 mm and NA =
0.25, Thorlabs C220 TME-C) was incorporated to collimate the beam from the laser through a
single-mode fiber (mode diameter 10.4 ± 0.8 µm). An achromatic doublet (f = 30 mm, Thorlabs
AC254) was used to refocus the beam to the back side of the sample. An identical lens is used
to collect the transmitted light. There are 3643 slots within the beam diameter, which was
measured to be 26µm using the knife-edge method.
16
2.4 Experiment
The photonic crystal device is mounted in a shallow PDMS microfluidic chamber. A thin, open
top PDMS microfluidic chamber (1 mm × 4 mm area, ~1 µm thickness) was fabricated on a
glass slide using standard photolithography methods. Tert-Butyl alcohol was used as a solvent
for the PDMS. The photonic-crystal sample was mounted on a circular glass slide with a 2 mm
circular hole at the centre and inserted in a rotary stage. The chamber was pressed firmly on to
the sample and sealed inside the rotary stage. Nanoparticles were injected into the chamber
through microfluidic tubes using a syringe pump. A constant, low velocity, laminar flow was
maintained in the chamber throughout the course of the experiment. Nanoparticle solutions
were prepared in heavy water (D2O, Sigma Aldrich) to minimize the thermophoresis effects
[78] resulting from water absorption around 1550nm. At this wavelength the absorption
coefficient of D2O is 27 times smaller than H2O [79].
Polystyrene nanoparticles with 520nm diameter (Thermo Scientific) are injected into the
chamber using a syringe pump. An erbium-doped fiber amplifier combined with polarization-
control optics were used to control the power and polarization of the beam. When the incident
laser is tuned to the wavelength of the guided-resonance mode, nanoparticles are attracted
toward the slab. We trapped a few nanoparticles on the slab at an incident power of 30mW and
determined the trap stiffness from the variance in particle position [69, 80]. 520nm particles
were used for the stiffness measurement, as they are clearly visible in the trapping videos and
have reasonably high diffusivity. It is difficult to detect the accurate position of the particles
when they are together in a cluster. In order to overcome this, a few isolated particles were
trapped by turning down the power. The particle positions were detected with subpixel
17
accuracy using the radial symmetry method [81]. The experiment was repeated, and position
data was collected for 62 particles over 1000 frames. The variances in particle position were
measured and corrected for motion blur due to the finite integration time of the camera and
detection error [82]. The stiffness values were normalized to the local power at each trapping
site. The maximum power per trap is estimated to be ~10.1 µW.
The measured variance was corrected for motion blur and tracking error [82]. The experiment
was repeated 15 times to collect the statistics. The distribution of trap stiffness is shown in Fig.
2.5.
Figure 2.5 Trap stiffness for incident y-polarized light. (a) Histogram of stiffness values in the direction
perpendicular to the polarization of the incident light. (b) Stiffness in the direction parallel to the incident
polarization
The mean stiffness in the x and y direction were found to be 52 pN/nm/W and 79 pN/nm/W
respectively. The stiffness in the y direction is higher than that in the x direction. This is
expected as the particles are mechanically confined in the y direction, due to their ability to
partially sink into the slots. In addition, the field confinement is stronger in the y direction than
18
in x. The stiffness is an order of magnitude greater than our previously demonstrated square
lattice photonic crystal design [69].
We carried out the trapping experiment for varied particle size and composition. Within the
illumination area, multiple particles were trapped. Some empty sites were observed, possibly
due to the inhomogeneties in slot dimension. We were able to trap dielectric particles with
diameters ranging from 300nm to 780nm and metallic nanoparticles with diameters ranging
from 250nm to 400nm. The ability to trap varied particle sizes and composition suggests the
near ‘universal’ character of slot-graphite traps. Figure 2.6 shows the optical microscope
images of a typical experiment. In these images, we used the maximum available optical power
in our setup (180mW) to trap as many particles as possible.
Figure 2.6 Trapping in slot-graphite lattice. Assembly of (a) 520nm polystyrene particles (b) 300nm gold
nanoparticles
To investigate the kinetics of particle trapping in the slot graphite optical lattice, we measured
the number of trapped particles as a function of time. Polystyrene particles with diameters of
380nm and 520nm are trapped separately with the same trapping power and particle
concentration. The results are shown in Fig. 2.7(a). The trapping process reaches a dynamic
equilibrium in which the number of trapped particles does not vary significantly. We observe
19
that the total number of trapped particles in equilibrium is similar for the two particle sizes,
while the initial cluster growth rate is higher for smaller particles.
We next carried out trapping experiments in a colloidal mixture containing equal
concentrations of 380nm and 520nm polystyrene particles. Figure 2.7(b) shows the
instantaneous number of trapped particles as a function of time. In equilibrium, almost all of
the trapped particles (94.3%) had a diameter of 380nm. The system thus shows a tendency to
selectively trap the smaller size particles.
In a colloidal mixture containing two types of nanoparticles, each species competes for the
available trapping sites in the lattice. The trapping sites are mutually exclusive: it is impossible
for two different particles to trap in the same site at the same time. The higher diffusivity of
smaller particles increases the probability of finding a smaller particle in the vicinity of a
trapping site than a larger one. Thus, the smaller particles occupy the available sites at a faster
rate. In addition, the smaller particles experience less drag, reducing the probability of trapped
particles being dislodged by the flow.
20
Figure 2.7 Instantaneous number of trapped particles for a colloidal solution containing equal concentrations
of (a) 380nm or 520nm polystyrene alone. (b) 380nm and 520nm polystyrene particles together.
A detailed study of the interplay between diffusion, drag and optical potential depth is the
subject of future work. In preliminary studies, we have used numerical simulations of the
Langevin equations to study broad trends. Our results indicate that trap selectivity can be
increased by increasing the flow speed of the injected particle solution, which increases the
differential drag between different sized particles. Meanwhile, the effect of differential
diffusivity on trap selectivity increases with chamber height.
21
2.5 Summary
In summary, we present a stiff, highly versatile near field optical trapping system which can be
used to optically assemble a large number of submicron nanoparticles with different sizes and
compositions. We demonstrate that the slot-graphite photonic crystal can selectively trap one
type of particles out of a mixture, acting as an efficient optical sieve.
Selective assembly is of great interest in trace analysis, optical diagnostics, enrichment, and
sorting of microscopic entities and molecules [83-86]. While size selective trapping of
microscale objects has been demonstrated in microfluidic systems [87-90], optical trapping
approaches eliminate the need for fluid flow [91], and are thus expected to enable a different
application range. Previously, surface plasmon based optical tweezers have been used for
selective optical manipulation [92]. Our traps offer an alternative dielectric solution.
Ultimately, we can envisage an arrayed, on-chip device for selective capture and parallel
detection of biomolecules attached to nanoparticles [93]
22
Manipulating secondary light interactions: Opto-
Thermophoretic Assembly
A version of the results in this chapter was published as Ref. [94]
3.1 Introduction
Optical trapping techniques provide powerful capabilities for the manipulation of nanoparticles
[2, 95, 96]. Recent work has used microphotonic devices to develop compact, low-power
optical traps for on-chip, microfluidic environments [35, 64, 66-68, 97, 98]. This includes the
development of parallel traps, capable of capturing many particles simultaneously [38-41, 69,
99]. Thermophoretic traps [100-103] meanwhile provide an alternative route to nanoparticle
manipulation, one based on the tendency of particles to migrate along thermal gradients.
Particles move either toward or away from heated regions, depending on the sign of their
thermophoretic coefficient [104, 105]. Trapping techniques that combine both optical and
thermal effects offer unique advantages [78, 106-111]. The ability to precisely manipulate
optical fields in space provides accurate, short-range control over location, while thermal
effects can facilitate long-range particle delivery.
In this work, we demonstrate that thermophoresis can be used as a tunable knob to adjust the
selectivity of a highly parallel microphotonic trap. Previous work in the literature has shown
23
that the magnitude and sign of the thermophoretic coefficient can be changed by modifying the
solution composition [112-114]. Here, we use this effect to tune the delivery of particles to
optical trapping sites. In particular, we demonstrate that we can tune the size selectivity of our
optical trap via the concentration of an added, non-ionic surfactant. In this manner, we can
choose to either select a single size particle out of a binary mixture, or to trap both sizes in the
mixture with equal probabilities. If desired, we can permanently immobilize the trapped
particles by polymerization of the trapping solution, providing a route to tunable fabrication of
nanoparticle films with controllable composition.
Figure 3.1 Schematic of optical trap based on photonic crystal. An array of optical traps is created upon
normal incidence illumination by a laser.
The schematic of our optical trap is shown in Fig. 3.1. A laser beam is focused on the surface
of a silicon photonic crystal slab. The photonic crystal is based on a slot-graphite pattern,
designed by introducing a narrow, rectangular slot in each unit cell of a conventional graphite
lattice [75]. The photonic crystal supports a guided resonance mode [43] around 1550nm. At
24
resonance, the electric field perpendicular to the slot is strongly enhanced due to the boundary
conditions [73, 74]. Subwavelength light confinement in the slots creates an array of stiff
optical traps, which can attract particles in solution [40]. This design provides an ordered array
of trapping sites, with a separation between sites (820nm) on the same order as the diameter of
the trapped particles (e.g. 300-780nm in Ref. [40]).
In addition to the optical trapping force, the particles can experience thermophoretic forces
and convection. Silicon has very low absorption at 1550nm. We further reduce absorption by
using heavy water as the trapping solution, which reduces the absorption by two orders of
magnitude relative to water [79]. We then leverage the residual absorption in the area of the
laser beam, and the associated local heating, to drive particle delivery to the trapping region.
3.2 Optical absorption in the trapping system
The optical absorption in the system is calculated using the 3D finite-difference time domain
(FDTD) method (Lumerical). The silicon photonic crystal is surrounded by heavy water. In the
simulation, we used an incident intensity of 1.75μW/ μm
2
. A 3D power monitor is deployed
around the high field intensity region to monitor the absorption in the system. The absorption
profile in x-y, y-z and x-z planes is shown in Fig. 3.2. It is evident that almost all the absorption
is happening in the surrounding medium. The losses in the silicon are negligible, in comparison.
25
Figure 3.2 Absorbed power in the unit cell of the photonic crystal. a) Coordinate system with origin at the
center of the slot, x axis along slot width and y axis along slot height. Power absorption along b) x-y plane c)
y-z plane d) x-z plane of the slot for an incident intensity of 1.75μW/ μm
2
.
3.3 Experiment
In a first set of experiments, we studied the effect of particle size on the net influx to the
trapping region. Fluoro-Max Dyed 380nm Green and 520nm Red Aqueous Fluorescent
Particles were purchased from ThermoFisher Scientific and diluted in heavy water to a particle
concentration of approximately 10
10
/cm
3
. The solution was introduced into a shallow (1.3µm
tall) PDMS reservoir in contact with the photonic crystal. The photonic crystal is mounted in a
sample holder perpendicular to the laser beam, which is parallel to the optical table. The
trapping laser power was measured to be 120mW, and the full width at half maximum (FWHM)
of the laser beam was estimated to be ~27µm using a knife edge measurement. A circular area
of the photonic crystal with the same radius as the beam was considered as the trapping region.
The concentration of particles in the trapping region was monitored with and without the
26
trapping laser, using an X-Cite 120Q light source and a 405nm bandpass filter, a 20X objective
lens (Mitutoyo), and a 5-megapixel CMOS monochrome camera (Imaging Source). Each
experiment was repeated three times under the same conditions, with time gaps of 15 minutes
between runs to allow complete heat dissipation. The concentration of particles in the trapping
region as a function of time was determined from processing the camera video, as in [40].
The temporal evolution of particle density in the trapping area is shown in Fig. 3.3. Figure
3.3(a) shows the results for an experiment with 380nm diameter particles. The thick green line
represents the average over experimental runs, and the shaded green region represents the
standard deviation. The grey region of the graph represents the time the laser was on. Prior to
laser turn-on, the particles diffuse freely due to Brownian motion. When the light is turned on
at 120s, the concentration increases and then stabilizes. When the light is turned off at 1200s,
the concentration drops. The spatial distribution of particles just before laser turn-off is shown
in the inset. Figure 3.3(b) shows the results for an experiment with 520nm particles. The
particle density in the trapping area drops when the laser is turned on, and it remains low until
the laser is turned off again. The inset shows that just before turn-off, there are no particles in
the trapping area. After turn-off, the density returns to its initial value. From this set of
experiments, we can conclude that the laser induces a net flow of particles into the trapping
region that is positive for 380nm particles, and negative for 520nm particles.
27
Figure 3.3 Particle density in the trapping region for (a) small, 380nm-diameter particles and (b) large,
520nm-diameter particles. The grey area represents the time during which trapping laser is on. The inset
shows the spatial distribution of particles in the active region before turning off the beam at 1200s; each green
circle labels a particle position
We next perform a trapping experiment with a mixture of 380 and 520nm particles. The
concentrations of the two sizes are equal. The height of the PDMS chamber is reduced to
800nm in order to provide better image clarity in the videos and allow us to distinguish between
the two particle sizes. The video is processed to count the number of trapped particles; a particle
is considered trapped if it moves less than one lattice constant between two adjacent video
28
frames. Figure 3.4 shows the number of trapped particles as a function of time after laser turn-
on.
Figure 3.4 Temporal evolution of optical trapping from a binary mixture of 380nm and 520nm particles.
Spatial distribution of the assembly from the mixture is shown in the inset. Small particles are shown in green
circles and large ones in red
The thick line shows the average over three runs, and the shaded region shows the standard
deviation. The number of trapped 380nm particles grows and then stabilizes. The number of
trapped 520nm particles is very low. The inset shows the distribution of particles at 275s.
Trapped particle locations are labelled by green and red symbols; almost all trapped particles
are 380nm in size. We conclude that in a mixed particle solution, only the smaller particles, for
which there is a net influx into the trapping area in single-size-particle experiments, are trapped
on the photonic crystal.
We can interpret the trends observed in Figures 3.3 and 3.4 by considering the sum of forces
on a single particle. The total force is the sum of three terms, 𝐹 𝑡𝑜𝑡 = 𝐹 𝑜𝑝𝑡 + 𝐹 𝑡 ℎ
+ 𝐹 𝑐𝑜𝑛𝑣 .
𝐹 𝑜𝑝𝑡 is the optical force, which is strongest near slots in the lattice and has an envelope
29
dependent on the incident Gaussian beam. 𝐹 𝑡 ℎ
is the thermophoretic force, and 𝐹 𝑐𝑜𝑛𝑣 is the drag
force on the particle due to convective flow. All three forces can potentially depend on particle
size. From Figure 3.3, we infer that the total force is attractive for smaller particles, and
repulsive for larger particles. We hypothesize that for the larger particles, a strongly repulsive
thermophoretic effect (𝐹 𝑡 ℎ
) prevents trapping.
The experiments above are performed in a sealed chamber. We can contrast the results with
our previous experiments in channels with steady flow [40]. In that case, for experiments
carried out with particles of single size, both sizes of particle trapped on the photonic crystal.
The moving fluid likely had two effects: to reduce the effects of thermophoresis by dissipating
heat at the beam spot, and to provide steady delivery of particles to the trapping sites. Either
effect will mitigate the effects of repulsive thermophoresis.
30
Figure 3.5 Particle density in the trapping area for (a) small 380nm particles and (b) large 520nm particles
with Triton-X. The grey area represents the time at which trapping laser is on. Spatial distribution of particles
in the active region before turning off the beam at 1200 s is shown in the inset
To trap both sizes of particle in our current experiment, without using fluid flow, we seek to
modify the strength of the thermophoretic effect. Previous work in the literature has
demonstrated changes in the thermophoretic coefficient due to addition of a surfactant [112-
114]. We perform a second set of experiments for single-size particle solutions, adding 0.5%
volume of Triton-X, a non-ionic surfactant. The results are shown in Fig. 3.5. With the added
surfactant, both 380nm and 520nm particles flow into the trapping region at laser turn-on. The
particle density for 520nm particles is slightly lower, but comparable to, the density for 380nm
31
particles. We can infer that the Triton-X has altered the thermophoretic coefficient of the larger
particles strongly enough that the thermal forces no longer prevent larger particles from
reaching the trapping region.
Trapping experiments performed with a binary mixture of particles sizes with Triton-X are
shown in Fig. 3.6. In this case, a mixture of 380nm and 520nm particles are trapped. The
number of trapped 380nm particles is slightly higher than the number of 520nm particles
trapped. We conclude that an increase in the net influx of larger particles into the trapping
region in single-size-particle experiments correlates with increased number of larger particles
trapped on the template.
Figure 3.6 Temporal evolution of optical trapping from a binary mixture of 380nm and 520nm particles with
Triton-X. Spatial distribution of the assembly from the mixture is shown in the inset. Small particles are
shown in green circles and large ones in red
We next show that by controlling the concentration of surfactant the solution, the trapping
selectivity can be tuned. Figure 3.7 shows the percentage of trapped particles with 380nm. As
the concentration is increased, the system can be tuned between only trapping smaller (380nm)
32
particle to trapping smaller and larger (520nm) particles in nearly equal amounts. The balance
between optical forces and thermal effects thus allows a tunable system, in which the size
distribution of trapped particles can be selected via surfactant concentration
Figure 3.7 Tuning the size selectivity of the optical traps by changing the surfactant concentration
3.4 Permanent Optical Assembly
The particles remain trapped on the photonic crystal lattice until the laser is turned off. To
permanently fix the particles in position, we used a photopolymerization technique [115]. A
UV curable polymer is introduced in the particle solution. 150µl of the particle mixture
containing 0.5% Triton-X is mixed with 50µl of photo curable polymer, Polyethylene glycol
diacrylate (PEGDA), and 0.01g of photo initiator, 2-Hydroxy-4′-(2-hydroxyethoxy)-2-
methylpropiophenone (Irgacure). Optical trapping is performed using the resonantly tuned
infrared laser. Once the particles are trapped on the photonic crystal, a 100W UV lamp is used
to illuminate the sample while keeping the trapping laser on. After the polymerization process,
33
both the UV lamp and trapping laser are turned off. The trapped cluster is preserved in the
polymerized matrix. Fluorescent images of the polymerized cluster are obtained using a 50X
Mitutoyo objective lens and a Nikon DS-Fi1color camera. Green 380nm particles are imaged
using a FITC excitation filter and a 535nm collection filter. Red 520nm particles are imaged
using a TRITC excitation filter and a 620nm collection filter.
Figure 3.8 shows the optical microscope images of the device before trapping, after trapping
and after polymerization. From the fluorescent image, we can see that particles of both sizes
have been trapped and immobilized in the polymer matrix.
34
Figure 3.8 Optical trapping and permanent immobilization. (a) Before Trapping. (b) Trapping laser is turned
on and trapping is completed. Trapped particles are exposed to UV light with trapping laser on. (c)
Polymerized matrix. Particles stay immobilized in the matrix even after the trapping laser is turned off. (d)
Fluorescent image of green 380 particles in the matrix. (e) Fluorescent image of red 520 particles in the
matrix
3.5 Conclusion
In conclusion, we have demonstrated that thermophoretic tuning provides a convenient tool for
controlling the selectivity of optical traps. Using a photonic crystal that supports a 2D array of
optical trapping sites and polystyrene particles with diameters of 380nm and 520nm, we
showed that adjusting surfactant concentration in solution tunes the size selectivity of trapping.
Our data suggests that tuning the surfactant concentration modifies particle delivery to the
trapping sites, by changing the strength of the thermophoretic effects. An interesting direction
for future research would be to determine whether tunable thermophoresis could be used in
other optical traps and for other objects, such as in optical traps specifically designed for
biological species [18, 68, 116-118].
35
We have further demonstrated that it is possible to permanently immobilize the trapped
particles on demand, via photopolymerization of the solution. This provides a method for
fabricating a solid film containing nanoparticles with a selectable degree of monodispersity.
The film can later be interrogated via fluorescent imaging or other post-processing analysis
tools, and/or used in applications.
36
Spatio-Spectral light management: Reconfigurable
Optical Assembly
A version of the results in this chapter was published as Ref. [119]
4.1 Introduction
Nanophotonics has opened up novel possibilities for the spatial and spectral control of light
[120, 121]. Integrated photonic devices can break the diffraction barriers of conventional optics
and confine light in sub-wavelength volumes [120-123]. The tightly-confined near fields of
dielectric photonic devices have been extensively used for optical trapping of microscopic
matter [35, 67, 68, 98]. Parallel trapping of multiple objects can be achieved by using devices
that support multiple, high-intensity spots in the near-field profile [38, 40, 69]. To develop
tunable or reconfigurable traps, mechanisms for changing the spatial periodicities of the near
field are desirable. Such a capability would allow real-time reconfiguration of “optical matter
[124],” the pattern of objects trapped on the device.
Typical approaches for reconfiguring the near field of a photonic device, such as mechanical
deformation, liquid infiltration, thermo-optic and nonlinear tuning [125-129], can pose
practical challenges for implementation within optical trapping systems. A relatively
straightforward route to reconfiguring the field is to design a device that supports different
37
spatial profiles at different wavelengths and/or polarizations within the tuning range of the
laser. In previous work, we have suggested this scheme theoretically for optical traps based on
guided resonance modes [43] of photonic-crystal slabs [130]. However, our initial design
required a laser tuning range much larger than is typical for lasers used in trapping experiments.
In this work, we propose, simulate, and demonstrate a method for designing closely-spaced
guided resonance modes in photonic crystals. We break the symmetry of an underlying
photonic-crystal lattice by introducing two sets of orthogonal slots. The slots create two modes
with different spatial field periodicities and polarizations. We show that the wavelength
separation between the modes can be made arbitrarily small by varying the lattice parameters,
while the center wavelength is adjustable. We implement our design experimentally in a silicon
photonic crystal near a wavelength of 1550nm. Our measurements demonstrate the ability to
adjust center wavelength and wavelength separation for two modes excited with orthogonal
linear polarizations. For optical trapping applications, our systematic design approach for dual,
closely-spaced modes can be used to create different optical potential landscapes for
reconfigurable trapping [130]. Our approach may also be useful for the development of
differential-mode biosensors [131, 132] and dual-band notch filters [133-137] in photonic
crystals.
4.2 Dual-slot photonic crystal design
As a starting point for our design, we consider a photonic-crystal slab comprised of a square
lattice of holes in silicon. This structure supports guided-resonance modes at the center of its
first Brillouin zone (𝛤 point) [43]. We set the lattice constant a = 642nm, the hole radius r =
38
90nm, the thickness of the Si photonic crystal slab to 250nm, and the refractive index to 3.45.
The slab lies on top of a semi-infinite SiO2 substrate with refractive index 1.44, and the
surrounding medium is assumed to have an index of 1.33 (e.g. water). The structure was
simulated using the 3D Finite-Difference Time Domain (FDTD) method in the Lumerical
FDTD Solutions tool. Fig. 4.1 shows the electromagnetic fields excited by normally-incident
light for a resonance mode wavelength of 1695nm. The in-plane electric field (𝐸 ̅
𝑥 𝑦 ) and
magnetic field component (|𝐻 𝑧 |) at the center of the slab are shown for x- and y-polarized light
in Figs. 4.1(a) and 4.1(b) respectively.
We next introduce slots into the square lattice. Due to the boundary conditions on the electric
field, a narrow slot in a dielectric will strongly enhance the electric field perpendicular to the
slot [74]. As shown in our previous works [40, 75, 130], this principle can also be used to
perturb guided-resonance modes. We introduce both horizontally and vertically-oriented slots
in the locations indicated by the white dashed lines in Fig. 4.1(a) and 4.1(b). These positions
are indicated schematically in Fig. 4.1(c). We call the slots oriented in the y direction (red
symbols) “center slots,” since they are placed at the center of each unit cell in the original
lattice. They form a square lattice with periodicity a. We call the slots oriented in the x-
direction (green symbols) “edge slots,” since they are placed along the edges between holes of
the original lattice. They also form a square lattice, but with a periodicity of √2 𝑎 , and a rotation
by 45
0
. By using different periodicities for the center and hole slots, we can obtain modes with
different near-field periodicities, as will be seen below. The slot dimensions are labeled
symbolically in Fig. 4.1(d).
39
Figure 4.1 a,b) In-plane electric field vector (arrows) and out-of-plane magnetic field component (color map)
at the center of the photonic-crystal slab formed by a square lattice of holes in silicon for (a) x- and (b) y-
polarized incident light, prior to the introduction of slots. c) Locations of center slots (red) and edge slots
(green) introduced into the square lattice. d) The design parameters of the dual-slot photonic crystal. a and r
are the lattice constant and hole radius of the original square lattice. The width and height of the center slots
are represented by w c, h c, and the edge slots by w e, h e.
Figure 4.2 shows the evolution of the guided-resonance mode in the square lattice as the slots
are introduced. The slot widths were set to we = w c = 250nm, while the slot height h = he = hc
= was varied from 0 to 80nm. The transmission spectrum is plotted in Fig. 4.2(a).
40
Figure 4.2 a) The transmission spectrum of the device for different slot heights. b-d) The magnetic (H z) and
electric field (|E|
2
) distribution at the center of the square lattice of holes for (b, d) x- and (c, e) y- polarized
light. f-i) The magnetic (Hz) and electric field (|E|
2
) distribution at the center of the dual-slot photonic crystal
for (f, h) x- and (g, i) y- polarized light.
The guided-resonance modes appear as dips in the spectrum. In the limit where the slots
disappear (h=0), the guided resonances are degenerate for both polarization states (top graph).
Introduction of slots into the design breaks the degeneracy. As the slot height increases, both
modes shift to lower wavelengths. For ℎ = 80nm, the x-polarized mode lies at λ1= 1535.30nm,
and the y-polarized mode at λ2= 1566.3nm, with a difference Δλ of 31nm. We note that incident
light with a linear polarization not aligned with the x- or y- axis will partially excite both modes,
yielding two dips in the transmission spectrum.
41
We compare the field distributions in the photonic crystal before and after the introduction of
slots in Figs. 4.2(b)-4.2(i). From Fig. 4.2(d) and 4.2(h), it can be seen that the introduction of
vertically-oriented (center) slots creates strong field confinement for the x-polarized mode.
Likewise, the introduction of horizontally-oriented (edge) slots creates strong field
confinement for the y-polarized mode (Fig. 4.2(e) and 4.2(i)). In addition, the x- and y-polarized
modes have distinct intensity distributions. The intensity distribution in Fig. 4.2(h) peaks at the
center of each center slot, while the intensity distribution in Fig. 4.2(i) peaks at the center of
each edge slot. The peak near-field intensity values corresponding to x- and y- polarizations
thus form square lattices with lattice constants of 642nm and 908nm, respectively.
The resonance wavelengths can be tuned by varying design parameters. This can be done by
modifying the underlying square lattice or by perturbing the slot dimensions. Figure 4.3 shows
the variation of x- and y- polarized modes as a function of the lattice constant a and hole radius
r of the photonic crystal. The slot dimensions were kept constant at 80nm and 250nm.
Figure 4.3 a) The resonant wavelength as a function of r and a for x- and y- polarizations. b) The difference in
resonant wavelengths for y- polarized and x- polarized modes as a function of r and a.
From Fig. 4.3(a), it is evident that both resonances are sensitive to the hole size and lattice
constant. The resonances redshift with the increase in lattice constant and blue shift with the
42
radius. The separation between the resonances varies with the radius and remains mostly
unaffected by the lattice constant (Fig. 4.3(b)). This gives us a convenient tool for controlling
the resonances. The relative locations of resonances can be controlled with the hole radii, and
then their absolute locations can be shifted with the lattice constant. We also investigated the
effect that the perturbation of slot dimensions has on the resonance wavelengths and quality
factors. The results are shown in Fig. 4.4.
Figure 4.4 The resonance wavelengths for x- and y- polarizations with a) width and b) height of edge slots, c)
width and d) height of center slots.
The lattice constant and hole radius are fixed at 620nm and 55nm so that both resonances are
close to 1550nm. Both the resonances blue shift with increase in slot sizes we, he, wc, and hc, as
is apparent from the negative slope in Figs. 4.4(b)-4.4(e). The changes in dimensions of the
edge slots shift both the resonances equally (Fig. 4.4(b) and 4.4(c)), while the center slots
change the relative locations of the resonances (Fig. 4.4(d) and 4.4(e)). The resonance
43
wavelengths are more affected by the center slots (Fig. 4.4(d) and 4.4(e)) than the edge slots
(Fig. 4.4(b) and 4.4(c)).
The quality factors of the resonances depend on the slot dimensions. The change in the quality
factors of the resonances with slot perturbation is shown in Fig. 4.5. The x-polarized mode has
orders of magnitude higher simulated quality factor than the y-polarized mode. The quality
factor of the x-polarized resonance is greatly affected by both the slots, but in opposite
directions. Increasing the width or height of the edge slots increases the quality factor of the x-
polarized mode, while the effect is reversed for the center slots. The quality factor of the y-
polarized resonance is marginally affected by the change in size of either of the slot types.
Increasing the size of the central slots and/or decreasing the size of the edge slots will reduce
the difference in quality factors of x- and y- polarized modes.
Figure 4.5 The quality factor of resonances for x- and y polarizations with a) width and b) height of edge
slots, c) width and d) height of center slots.
44
From the simulation results, the slot dimensions can be used to change the quality factors of
the resonances. By expanding the central slots and shrinking the edge slots, the quality factor
of the two resonances can be made more similar. The corresponding change in locations of the
resonances can be compensated by adjusting the lattice constant and hole dimension.
4.3 Fabrication and characterization
The photonic crystal devices are fabricated on a silicon-on-insulator wafer from SOITEC with
a 250nm Si device layer, 3μm buried oxide layer, and a 4500μm Si handle layer. The patterns
are transferred to the wafer using e-beam lithography (Vistec EBPG 5000+ES, 100kV
acceleration voltage) followed by ICP-reactive ion etching (Oxford) with ZEP 520A resist. A
200nm thick Si3N 4 antireflection layer is coated on the backside of the polished wafer using
plasma enhanced chemical vapor deposition (Oxford). The lattice constant a and the hole radius
r of the devices were tuned from 644nm to 668nm and 65 nm to 101nm respectively, by keeping
the slot dimensions constant. The slot dimensions we, he, wc, hc were chosen as 250nm, 80nm,
275nm and 100nm respectively. The central slots were chosen to be slightly larger than the
edge slots to have comparable quality factors. The SEM image of one of the fabricated devices
is shown in Fig. 4.6.
45
Figure 4.6 SEM image of one of the devices. The scale bar represents 1µm.
The transmission spectra of the devices were characterized at normal incidence in parallel
polarization-mode by orienting polarizers before and after the sample in parallel. A Santec
TSL-510 tunable laser (1500-1620nm) was used for the characterization. The power and
polarization of the incident beam were controlled using an erbium-doped fiber amplifier
combined with polarization-control optics. An aspherical lens (f = 11 mm and NA = 0.25,
Thorlabs C220 TME-C) was used to launch the laser from a single-mode fiber (mode diameter
10.4 ± 0.8 μm) to free space. An achromatic doublet (f = 30 mm, Thorlabs AC254), mounted
on a lens tube, was used to refocus the beam to the back side of the sample. A second lens was
used to collect the transmission. The FWHM of the beam was measured to be 27μm using the
knife edge method.
Figure 4.7(a) shows the measured transmission spectra for x- and y- polarizations for varying
lattice constants, for fixed hole radius of 65nm. Each polarization state selectively excites one
of the guided modes. Both x and y polarized resonances red shift with the increase in lattice
constant, while the separation between the resonances remains nearly constant. By varying the
46
lattice constant, the resonances can be spectrally shifted to the left or right without affecting
their separation.
The measured transmission spectra for devices with varying hole radii is shown in Fig.4.7(b),
for fixed lattice constant of 644nm. The increase in hole size blue shifts both resonances. The
x-polarized resonance shifts more strongly than the y-polarized one, reducing the separation
between the individual resonances. For the largest radius used here, the two resonances nearly
coincide.
Figure 4.7 Transmission spectrum of dual-slot devices for a) different lattice constants and b) different hole
radius.
47
Figure 4.8 shows the experimental quality factors of the measured devices and compares with
simulation. The measured quality factors are lower than in simulations, a common effect of
slight fabrication errors and/or surface roughness [75]. However, the x-polarized mode has
higher Q than the y-polarized mode, as predicted by the simulation results.
Figure 4.8 Quality factor of the fabricated devices as a function of the a) lattice constant and b) hole radii.
Circles denote the measured Q; stars denote the value of Q from simulation.
4.4 Conclusion
We have proposed and experimentally demonstrated an approach to designing photonic-crystal
slabs that support closely-spaced guided-resonance modes with orthogonal linear polarizations.
The use of slots to break the symmetry of the underlying photonic-crystal lattice produces both
strong field confinement and modes with different spatial periodicities. The absolute and
relative location of the modes can be tuned by varying the design parameters, which we verify
via experiments in a silicon photonic-crystal slab at 1550nm. We envision that our design
approach can be used for reconfigurable optical trapping of nanoparticles with different spatial
separations and orientations [130], as well as in polarization-dependent filters and differential-
mode sensors. More abstractly, a difference in the spatial field profiles of multiple, closely-
spaced resonances of the same device could also prove useful in other applications that rely on
48
intensity-dependent nonlinearities and/or phase transitions [138]. Future designs may benefit
from the use of multiple sets of slots with different lattice constants and/or dimensions.
49
Stabilizing Optical Forces: Laser Sail
A version of the results in this chapter is submitted for publication.
5.1 Introduction
The proposed use of radiation pressure to propel objects in space has long been a subject of
fascination [1-4]. Both solar illumination and laser light have been considered as potential light
sources [1-10]. Most recently, the Breakthrough Starshot Initiative has proposed the bold goal
of sending a laser-propelled sail to Alpha Centauri to collect scientific data [11, 12]. Some of
the major challenges and criteria for the laser sail design have previously been summarized in
Ref. [13]. Here, we focus on the crucial issue of beam distortion. Since the Breakthrough
project plans to use a powerful, ground-based laser system as the light source, the incident light
on the laser sail will be distorted by travel through the atmosphere [14-20]. Such beam
distortions may dramatically impact sail stability, causing unwanted torque and deflecting the
sail trajectory. In this paper, we propose and analyze the use of a nonlinear optical material for
the sail to help passively stabilize against beam distortions.
The effect of beam distortion on sail stability is illustrated in Fig. 5.1. Suppose the ideal beam
intensity is symmetric around the sail center (blue line in Fig. 5.1(a)). Beam distortions will
disturb the intensity, potentially resulting in asymmetric profiles (e.g. red line in Fig. 5.1(a)).
50
For a linear sail material, the reflected power is proportional to the incident power (blue, dashed
line in Fig. 5.1(b)). As a result, the radiation pressure will also be asymmetric with respect to
the sail center (red, dashed line in Fig. 5.1(c)). This can create a torque on the sail and result in
spinning (indicated schematically by the arrows). Spinning may derail the sail trajectory, a
problem that is made particularly acute by the lack of restoring forces in the vacuum of outer
space.
Beam distortion is a common problem in astronomy, where adaptive optics solutions are used
to correct images for atmospheric perturbations [21-25]. Given real-time information about the
effect of the atmosphere on the beam spot, the wave fronts from the ground-based laser might
in principle be pre-compensated (e.g. deliberately distorted) so as to yield a uniform,
undistorted spot on the sail. However, given the extremely high target speeds to be attained by
the sail (0.2c at the end of acceleration) and the fundamental limits on the response time of
adaptive optics introduced by the travel time of light between the atmosphere and the ground
(~ milliseconds), adaptive optics alone may not be sufficient to ensure stability. It is thus of
interest to consider how the sail itself might be designed to provide stabilization against
residual beam distortions.
51
Figure 5.1 a) Intensity profile of incident laser beam at the sail surface: ideal (red) and perturbed (blue). b)
Reflected power as a function of incident power from a linear sail and a nonlinear sail. c) Radiation pressure
distribution across the sail. An asymmetric distribution results in unwanted torque.
To reduce the effects of beam distortion on sail stability, we introduce a nonlinear material for
the sail. In particular, we propose to use a nonlinear photonic crystal (PhC) to provide a
relatively flat reflected power over a range of incident powers. This property is depicted
schematically in Fig. 5.1(b) (green line). For the nonlinear PhC, the radiation pressure across
the sail will also be flattened, as shown in Fig. 5.1(c). This reduces asymmetry, and hence
unwanted torque. In Section 5.2 below, we analyze the optimal characteristics for the PhC
design within a semi-analytical approach. We then present a specific PhC design
implementation in Section 5.3 and analyze its performance. We show that the nonlinear PhC
achieves high stability while maintaining high overall thrust, in contrast to comparison linear
systems.
52
5.2 Effect of Nonlinear Resonance on Stability and Thrust
Previous work has shown that a photonic-crystal (PhC) slab structure, consisting of a high-
index, finite thickness layer patterned with a periodic array of holes, supports electromagnetic
modes known as guided resonances [42, 43]. A characteristic feature of these modes is that the
transmission and reflection spectra exhibit Fano lineshapes at resonance wavelengths. A
mathematical formula for the reflection near a guided-resonance mode is given by temporal
coupled-mode theory as [139],
( ) ( )
( )
2
2
22
00
2
2
0
11
2
1
ref
in
r t rt
P
P
− + −
=
−+
+
(5.1)
where r ,t are real-valued parameters representing the reflectivity and transmissivity of the
unpatterned slab,
0
is the resonance frequency, and is the cavity lifetime. For a Kerr
nonlinear system [140], the resonance frequency shifts with increasing input power as,
( )
3
0
0
2
2
0
1
1
in
res
P
P
=−
−+
(5.2)
Where
res
is the resonance frequency in the absence of nonlinearity (i.e. limit of low incident
power).
0
P is a characteristic power given by [141]
0
2
2
2
1
res
P
kQ n
c
=
(5.3)
53
where
2
n is the Kerr coefficient, /2
res
Q = is the quality factor of the resonant cavity, and k
is the nonlinear feedback factor that defines the degree of the spatial confinement of the mode
in PhC slab,
( ) ( ) ( ) ( ) ( ) ( )
( ) ( ) ( )
2 2
3 * 2
3
2
2
3 2 2
2
. 2 .
||
Vol
max
Vol
d r E r E r E r E r n r n r
c
k
d r E r n r n r
=
+
(5.4)
From Equations 5.1 through 5.4, we see that as the intensity of light incident on the slab
increases, the resonance frequency shifts as well. The resulting shift in the reflection spectrum
results in a nonlinear dependence of reflected power on incident power. Below, we study how
the nonlinear dependence can be controlled by adjusting the initial wavelength, quality factor,
and line-shape of the resonance.
For concreteness, we start by considering a quality factor Q = 5000, and r = -0.18. We assume
that
2
1 tr =− (no intrinsic material absorption). We define the detuning
in res
= − as the
difference between the laser wavelength ( 1064nm)
in
= and the wavelength of the resonance in
the absence of nonlinearity, 2/
res res
c = .
54
Figure 5.2 a) Intensity profile of incident laser beam at the sail surface: ideal (solid) and actual (dashed). b)
Reflected power as a function of incident power from linear material and nonlinear photonic crystal. c)
Radiation pressure distribution across the sail. An asymmetric distribution results in unwanted torque.
The reflection spectrum is shown in Fig. 5.2(a) for 0 = . The red curve plots the resonance in
the low-power limit. The reflection is strongly peaked as a function of wavelength, as is
characteristic of a resonance mode. As the input power increases, the spectrum shifts to the
right due to the modification of the resonance wavelength by the Kerr nonlinearity. The
resonance is initially centered at the input laser wavelength ( )
in res
= . As the input laser power
increases, the reflection spectrum shifts to higher wavelengths. This results in a change in
reflectivity at the laser wavelength, as indicated by the circles in Fig. 5.2(a).
The shift in the reflection spectrum is nonlinear with laser power and strongly depends on the
initial detuning of the laser wavelength from the resonance, as shown in Fig. 5.2(b). For the
55
zero-detuning case, the shift is monotonic with laser power (yellow line). The plot also shows
several cases of non-zero detuning. For sufficiently large, positive detuning (e.g. Δ = 0.2nm),
the curve becomes multivalued; there is a region of Pin/Po for which more than one solution for
Δλres exists. However, for sufficiently large values of Pin/Po (i.e. > 2 in the figure), all curves
shown are single valued and monotonically increasing, and the shift increases with increasingly
positive detuning.
The reflectivity at the laser wavelength is shown in Fig. 5.2(c). Starting with the zero-detuning
case (yellow line), the reflectivity decreases with increasing laser power, as expected from the
symbols in Fig. 5.2(a). For negative detuning, the reflectivity also decreases monotonically.
For positive detuning, the reflectivity first increases, and then decreases, with increasing power.
The nonmonotonic behavior results from the shifting of the resonance peak through the laser
wavelength. For all curves shown, sufficiently large values of Pin/Po yield decreasing
reflectivity with increasing power.
The reflected power as a function of input power is shown in Fig. 5.2(d). We will refer to this
function as the input-reflected power curve. The reflected power increases and then flattens as
a function of input power for all values of detuning shown. We will focus on a range of input
powers for which all curves are relatively flat, indicated by the grey, shaded box (3 < Pin/Po <
4). In this region, increase in input power is largely compensated by the reduction of reflectivity
shown in Fig. 5.2(c). As a result, the reflected power is nearly constant. Fig. 5.2(d) thus
illustrates the key feature of our approach: by designing the sail to support a guided resonance
mode and a Kerr nonlinearity, we achieve a relatively constant reflected power over a range of
input powers. This effect will mitigate the type of instability illustrated in Figure 1, which
56
results from a linear increase in reflected power with input power. We note that Fig. 5.2(d) also
shows that the reflected power increases with positive detuning, increasing the thrust on the
sail.
Figure 5.3 examines how the flatness and amplitude of the reflected power depend on the
quality factor of the resonance, for various values of detuning. To quantify flatness, we
calculate the average value of dPref/dP in over an input power range from 3P0 to 4P0 (e.g. the
average slope of the input-reflected power curve in Fig. 5.2(d)). The results are shown in Fig.
5.3(a). We exclude the region of parameter space where the reflected power curve is
multivalued between 3P0 to 4P0, indicated by the grey region in the plot. An average slope of
zero is obtained for values of detuning and quality factor lying along the dashed, green line.
This line thus indicates the optimal condition for sail stability. As Q increases, the optimal
detuning decreases as well. Fig. 5.3(b) shows the averaged reflected power. For a given value
of Q, the average reflected power increases with detuning. Operating along the maximum
flatness curve (green, dashed line) yields a relatively high value of reflected power, though not
maximum. Since the thrust on the sail is proportional to the reflected power, the region of
parameter space lying above the dashed line and below the grey region represents a trade-off
between maximum stability and maximum thrust.
57
Figure 5.3 a) Average slope of the input-reflected power curve. b) Average reflected power. Regions of
parameter space where the curve is multivalued are excluded from the plot (indicated by grey region).
The choice of Q will determine the required input power for operation. Since P0 is proportional
to 1/Q
2
from Eq. 5.3, using a higher-Q structure will require less input power from the laser to
shift the system into the nonlinear regime shown in Fig. 5.2(d), e.g. to ensure that Pin > 3P0.
The shape of the resonance used (e.g. the red curve in Fig. 5.2(a)) affects the nonlinear behavior
of the reflected power. Fig. 5.4(a) shows the reflection spectrum in the limit of low power for
different r values. The quality factor is fixed at 5000, and t is equal to 1 - r
2
. The spectrum is
completely symmetric for r=0. Nonzero values of r make the spectrum asymmetric. Focusing
on wavelengths below the resonance peak (λ < 1064nm), reflectivity decreases with decreasing
r. This tends to make the reflectivity fall off faster with input power, as the Kerr nonlinearity
shifts the resonance peak to higher wavelengths. Fig. 5.4(b) shows the input-reflected power
curve for zero detuning. As expected, the flattest response is obtained for r = -0.2. Increasing
r thus reduces sail stability. There is a trade-off between stability and thrust; decreasing r
reduces the value of Pref/P0.
58
Figure 5.4 a) Average slope of the input-reflected power curve. b) Average reflected power. Regions of
parameter space where the curve is multivalued are excluded from the plot (indicated by grey region).
5.3 Design Implementation of Light Sail
To implement our approach, we have designed a photonic-crystal slab that supports a nonlinear
guided-mode resonance with the required characteristics. We consider a silicon nitride slab
patterned with a square lattice of air holes. Silicon nitride was selected due to its combination
of significant Kerr nonlinearity and negligible absorption at wavelengths close to 1064nm. The
hole radius r = 0.22a, thickness t = 0.76a, and lattice constant a = 1µm were chosen to place
a guided-mode resonance close to 1064nm. We simulated the reflection spectrum for normally-
incident light using Lumerical FDTD solutions, using the wavelength-dependent refractive
index data found in Ref. [142]. In particular, we obtained the reflection spectrum in the low-
power limit by setting the Kerr coefficient to zero in the FDTD simulation.
The low-power reflection spectrum is shown in Fig. 5.5(b). From the figure, we see that the
slab supports a Fano resonance with a quality factor Q = 5217 at 1064.1nm and a peak
reflectivity close to 1. Fitting the spectrum to the Fano lineshape gives r = -0.15 and t = 0.96.
59
Figure 5.5 a) Schematic of silicon nitride photonic crystal slab consisting of a square lattice of air holes of
radius 0.22a, where a = 1µm is the lattice constant, and a thickness of 0.76a. b) Reflection spectrum of the
structure. c) Dependence of reflected power on incident power.
To calculate the nonlinear response of the photonic crystal, we use Eq. 5.1 along with the fitted
values of the resonance parameters. The result is shown in Fig. 5.5(c). It is evident from the
figure that the photonic crystal demonstrates a flattened input-reflected power curve for power
levels above 3P0. To find the physical value of the characteristic power Po, we compute the
nonlinear feedback factor using Eq. 5.4 and the electric field obtained from the FDTD
simulation. Taking n2 = 10
-18
m
2
/W [143], we find that k ≈ 0.014. This yields an estimate of P 0
= 0.076W per unit cell, or 76GW/m
2
for a lattice constant of 1µm. The incident laser used to
propel the Laser Sail is planned to operate at powers on the order of 100GW/m
2
[144].
To provide insight into how our nonlinear approach benefits sail performance, we compare our
design to several comparison cases. For linear systems, we find that increased stability
60
necessarily comes at the cost of decreased thrust. The nonlinear system breaks this constraint,
providing both high stability and high thrust simultaneously.
We first consider a perfect mirror (R = 1). Fig. 5.6(a) shows the average slope of the input-
reflected power curve as a function of detuning from resonance. Low average slopes
correspond to high stability. Fig. 5.6(b) shows the normalized, average reflected power. High
values correspond to high thrust. Both quantities are constant and equal to 1 (green, diamonds).
The perfect mirror thus provides low stability and high thrust. This behavior results from the
input-reflected power curve shown in Fig. 5.6(c), which is linear with slope 1.
The case of a linear, dielectric slab with the same thickness and linear refractive index as our
design is shown by the yellow stars in Fig. 5.6(a) and 5.6(b). The average slope is equal to the
normalized, average reflected power, as is true for any linear system. Both quantities are low
relative to the perfect mirror, corresponding to high stability and low thrust. In Fig. 5.6(c), this
case corresponds to a much lower-lying and flatter line than for the perfect mirror.
61
Figure 5.6 a) Average slope of the reflected power vs. input power curve and b) average reflected power for
different detuning. c) Reflected power response of linear materials and nonlinear photonic crystal.
Patterning the linear slab with holes, in the same pattern as our design, creates a guided-
resonance mode. Since the Kerr coefficient is set to zero, the resonance does not shift with
input power. The dependence of average slope and normalized, average reflected power on
detuning (shown by blue squares in Fig. 5.6(a) and 5.6(b)) is determined entirely by the line
shape shown in Fig. 5.5(b). Both quantities are identical and decrease with detuning.
Depending on the value of detuning chosen, this structure can provide either low stability and
high thrust, or high stability and low thrust. However, high stability and high thrust cannot be
obtained simultaneously. Visually, this trade-off is illustrated by Fig. 5.6(c). Changing the
detuning shifts the input-reflected power curve between that of the mirror and that of the linear
slab. Increased stability (flatter line) comes at the cost of decreased thrust (lower reflected
power).
62
The nonlinear system breaks the fundamental constraints of the linear system to simultaneously
provide high stability and high thrust. As shown by the red dots in Fig. 5.6, the average slope
(Fig. 5.6a) approaches zero at a detuning of approximately 0.25nm. The normalized reflected
power meanwhile increases with detuning as shown in Fig. 5.6b. Both high stability (small
average slope) and high thrust (large normalized, average reflected power) are obtained by
using a detuning close to 0.25. With this choice, the overall performance of the nonlinear
guided-resonance mode design is superior to all the comparison cases. The input-reflected
power curve is shown in Fig. 5.6(c). This choice of detuning provides a flat line at high reflected
power.
5.4 Conclusion
In conclusion, we have proposed a passive solution for increasing the stability of a laser-
propelled sail in the presence of intensity distortions in the incident beam. The key concept of
our approach is to design the sail to exhibit a flattened input-reflected power curve at the laser
wavelength. In particular, we achieve this goal using a guided-resonance mode in a Kerr
nonlinear photonic crystal. We used semi-analytic coupled mode theory to analyze the
characteristics of the mode that yield the flattest curve. We presented a concrete design based
on a silicon nitride photonic crystal that supports a mode with the desired characteristics. We
compared the performance of our nonlinear design with designs based on linear materials. For
linear systems, the linear dependence of reflected power on incident power imposes a trade-off
between stability and thrust on the sail. We show that our nonlinear design lifts this constraint,
simultaneously providing both high stability and high thrust.
63
Throughout our analysis, we have assumed a fixed operating wavelength of 1064nm for the
incident laser. Under the anticipated flight conditions, the sail will experience a Doppler shift
from the incident laser [145]. Future work will consider how the approach proposed here can
be extended to incorporate resonance-tuning and/or multiple-resonance designs to leverage the
stabilizing effects of nonlinearity over extended wavelength ranges.
64
Conclusions and Outlook
6.1 Conclusion
In PART I of this dissertation, we used the optical forces in nanophotonic structures for
nanomanipulation and optical propulsion. The main contribution and discoveries of PART I of
this dissertation can be characterized as following.
First, I had designed, fabricated and characterized a slot graphite photonic crystal that support
a guided resonance mode in the telecom wavelength. The enhanced trapping performance of
the photonic crystal was demonstrated. The versatility of the template in assembling submicron
particles of various sizes and composition was also demonstrated. Then I had shown that the
thermophoretic forces in the system can be used as a convenient tool for controlling the
selectivity of optical traps. Using a photonic crystal that supports a 2D array of optical trapping
sites and polystyrene particles with diameters of 380nm and 520nm, we showed that adjusting
surfactant concentration in solution tunes the size selectivity of trapping. Our data suggests that
tuning the surfactant concentration modifies particle delivery to the trapping sites, by changing
the strength of the thermophoretic effects.
I had further demonstrated that it is possible to permanently immobilize the trapped particles
on demand, via photopolymerization of the solution. This provides a method for fabricating a
solid film containing nanoparticles with a selectable degree of monodispersity. The film can
65
later be interrogated via fluorescent imaging or other post-processing analysis tools, and/or
used in applications.
The ability to change the arrangement in the self-assembly as desired can ultimately lead to the
development of reconfigurable optical matter. Designing multiple modes in the same trapping
template with different spatial field distributions would be huge step towards the development
of this field. We have proposed and experimentally demonstrated an approach to designing
photonic-crystal slabs that support closely-spaced guided-resonance modes with orthogonal
linear polarizations. The use of slots to break the symmetry of the underlying photonic-crystal
lattice produces both strong field confinement and modes with different spatial periodicities.
The absolute and relative location of the modes can be tuned by varying the design parameters,
which we verify via experiments in a silicon photonic-crystal slab at 1550nm. We envision that
our design approach can be used for reconfigurable optical trapping of nanoparticles with
different spatial separations and orientations well as in polarization-dependent filters and
differential-mode sensors.
Finally, we had explored the use of optical forces in nanophotonic structures for macroscopic
applications. We have proposed a passive solution for increasing the stability of a laser-
propelled sail in the presence of intensity distortions in the incident beam. The key concept of
our approach is to design the sail to exhibit a flattened input-reflected power curve at the laser
wavelength. In particular, we achieve this goal using a guided-resonance mode in a Kerr
nonlinear photonic crystal. We used semi-analytic coupled mode theory to analyze the
characteristics of the mode that yield the flattest curve. We presented a concrete design based
on a silicon nitride photonic crystal that supports a mode with the desired characteristics. We
66
compared the performance of our nonlinear design with designs based on linear materials. For
linear systems, the linear dependence of reflected power on incident power imposes a trade-off
between stability and thrust on the sail. We show that our nonlinear design lifts this constraint,
simultaneously providing both high stability and high thrust
6.2 Outlook
An interesting direction for future research would be to further study and use the differential
forces in the optical trapping systems in biological applications. Selective assembly is of great
interest in trace analysis, optical diagnostics, enrichment, and sorting of microscopic entities
and molecules. While size selective trapping of microscale objects has been demonstrated in
microfluidic systems. optical trapping approaches eliminate the need for fluid flow and are thus
expected to enable a different application range. Previously, surface plasmon based optical
tweezers have been used for selective optical manipulation. Our traps offer an alternative
dielectric solution. Ultimately, we can envisage an arrayed, on-chip device for selective capture
and parallel detection of biomolecules attached to nanoparticles. Experimentally demonstrating
the reconfigurable optical assembly is another work for the future. We can envision optically
assembled metasurfaces which can be reconfigured to support multiple optical functionalities.
67
Part II
Thermal emission in Nanophotonic structures
68
Introduction
7.1 Tailoring thermal emission
Thermal emission is one of the fundamental aspects of nature. Every object at a finite
temperature emits electromagnetic radiations through radiative relaxation of thermally excited
particles to lower energy levels [146]. Radiation from conventional thermal emitters are usually
broadband, incoherent, unpolarized and isotropic [147]. For an object in thermal equilibrium
with its surroundings, it’s thermal emissivity will be equal to absorptivity for all wavelengths,
directions and polarization [148]. This is called Kirchhoff’s law which is mathematically
expressed as: [147]
( , , ) ( , , ) n p e n p = (7.1)
where ( , , ) np is the angular spectral absorptivity and ( , , ) e n p is the angular spectral
emissivity of the object. The angular spectral absorptivity represents the absorption coefficient
of the structure for incident light at a frequency and direction n with a polarization vector
p , and is typically measured by taking the ratio between the incident and the absorbed power
per unit area. The angular spectral emissivity measures the spectral emission power per unit
69
area at a frequency , to a plane wave propagating at the direction n with a polarization vector
p normalized against the spectral emission power per unit area of a blackbody emitter at the
same frequency to the same direction [147].
From Kirchhoff’s law, the thermal emission intensity spectrum of a real object or a surface at
temperature T is equal to the absorbance () A weighted by the blackbody distribution
spectrum ( , )
BB
IT [149, 150].
Re
( , ) ( ) ( , )
al BB
I T A I T = (7.2)
The blackbody radiance is given by Planck’s law of thermal radiation as: [151]
2
5
21
( , )
1
B
BB hc
kT
hc
IT
e
=
−
(7.3)
where c is light speed, h is the Planck’s constant, and k B is the Boltzmann’s constant.
According to Kirchhoff’s radiation law, the thermal emission spectrum can be modified by
tailoring the absorptivity of the target material [152]. This offers many exciting opportunities
in the areas of applied physics and engineering [148]. For example, converting from a
broadband to a narrowband thermal emission spectrum with minimal loss of energy is
important in the creation of efficient environmental sensors and biosensors as well as thermo-
photovoltaic power generation systems [153].
70
Subwavelength optical structures can be used to modify the absorptivity of materials towards
externally introduced light. The wave interference effect in these structures can produce
thermal radiation drastically different than the underlying materials. They could produce
coherent, narrowband, polarized, enhanced and directional thermal radiation [147].
Dynamically tunable thermal radiation can also be produced using subwavelength structures,
where the emission intensity can be modulated as a function of time. Various strategies have
been employed to introduce dynamically control the emission response [150, 154, 155]. One
such method is combining the nanophotonic structures with phase change materials [156-160].
In Part II of this thesis, dynamic manipulation of thermal emission using nanophotonic
structures is discussed. The phase transition in the vicinity of the resonant nanophotonic
structures can perturb the resonance and consequently the emission response.
7.2 Surface Plasmons in nanostructures
Surface plasmons are light waves trapped and propagate along the surface of conductors. They
originate because of resonant interaction between the surface charge oscillation and the
electromagnetic field of the light. The field perpendicular to the surface decays exponentially
away from the surface. Therefore, the surface plasmons help to concentrate and channel light
using subwavelength structures. The interaction between surface charges and electromagnetic
wave results in the momentum of the surface Plasmon mode (ℏ𝑘 𝑠𝑝
) to be higher than that of
the electromagnetic wave (ℏ𝑘 0
). The frequency dependent wave vector of surface plasmon
mode can be obtained, by solving the Maxwell equations, as [161]
71
0
md
sp
md
kk
=
+
(7.4)
Where,
m
and
d
are the frequency dependent complex permittivity of the metal and
dielectric. The momentum mismatch demands some coupling mechanism for the free space
photons to excite the surface plasmon modes. There are three major techniques popularly used
for the momentum matching. They involve prism coupling, scattering from a topological defect
on the surface and periodic corrugation in the metal’s surface [161]. The evanescent fields
associated with the excited surface plasmon polariton waves will penetrate the dielectric and
the metal. The skin depth in dielectric and metal are given by
2
0
1
| ( ) / |
d d m d
k
=+ and
2
0
1
| ( ) / |
m d m m
k
=+ respectively.
Surface plasmons in bound geometries surface such as metallic particles are of non-propagating
nature and are called localized surface plasmons (LSPs). The curved surface of the bounded
geometries generates an effective restoring force on the driven electrons, resulting in resonance
occurring on the surface, called localized surface plasmon resonance (LSPR). The LSPs
(LSPR) can be excited by direct light illumination, whereas the SPPs can be excited by
matching the frequency and the momentum between the excitation light and the SPPs [162].
7.3 VO2 phase change material
Vanadium dioxide (VO2) exhibits a strongly correlated electron system, which exhibits a
structural phase transition [163]. At temperatures below Tc, VO2 has a semiconducting
72
monoclinic structure. Above Tc, the structure changes to metallic tetragonal, like rutile TiO2.
The phase transition dramatically changes the optical and electrical properties of VO2 in a
reversible manner. Figure 7.1 shows the change in structure and refractive index of VO2 before
and after the phase change
Figure 7.1 a) The crystal structure of VO 2 below and above phase transition temperature. The refractive index
of VO 2 when the material is in b) insulating and c) metallic state
The temperature dependent optical properties of VO2 can be modelled as a superposition of
insulating and metallic states as: [164]
2
( ) ( ) (1 ( ))
VO ins met
T f T f T = + − (7.5)
Where, 𝜀 𝑉 𝑂 2
(𝑇 ) is the refractive index of VO2 as a function of temperature, 𝜀 𝑖𝑛𝑠 and 𝜀 𝑚𝑒𝑡 are
the dielectric permittivity of the insulating and metallic phases, and
( ) 1/ 1 exp
c
c
TT
fT
WT
−
=+
(7.6)
73
is a temperature-dependent function governing the distribution of insulating and metallic
optical properties (𝑇 𝑐 is the transition temperature and 𝑊 is variable which controls the width
of the transition).
The change in optical properties of VO2 with temperature has been extensively used for
thermochromic applications [165, 166], thermally tunable metamaterials [167, 168], Optical
switches [169], passive thermal homeostasis devices [170, 171], etc. Core shell VO2
nanoparticles has also been theoretically proposed for self-thermoregulation [172]. When
nanostructures are combined with VO2, they can enhance the phase transition of the VO2
material. Switching response in the range of picoseconds [173] and nanoseconds [174] are
reported in VO2 with optically heated nanostructures. The transition temperature and width of
the temperature hysteresis can be altered via doping [175] or strain engineering [176].
74
Thermally switchable narrowband emission using
phase change materials
A version of the results in this chapter is submitted for publication.
8.1 Introduction
The ability to manipulate thermal emission from materials can have numerous applications,
including infrared sensing [177, 178], thermal imaging [179, 180], homeostasis [170],
camouflage [181], infrared sources [182] and thermophotovoltaics [183-186]. The emitted
radiation from conventional thermal emitters are typically incoherent, broadband, un-polarized,
and the emission pattern is near isotropic [147, 187]. But most applications require only a
narrow region of the emission spectrum [188, 189]. For example, converting from a broadband
to a narrowband thermal emission spectrum with minimal loss of energy is important in the
creation of efficient thermo-photovoltaic power generation systems and environmental sensors
[153]. Dynamically controlling the emission properties without structural change enables the
modulation the thermal emission [150, 189]. Therefore, there is a strong motivation to realize
switchable narrowband thermal emission with high peak emissivity in engineered materials.
One way to break the fundamental constraints on thermal emission is to use engineered
nanostructures [190-192]. The wave interference effects in subwavelength structures lead to
75
thermal emission spectra drastically differ from underlying materials [147]. Various strategies
have been employed to introduce dynamically control the emission response in nanophotonic
structures [150, 154, 155]. One such method is combining the nanostructures with phase
change materials [156-160]. The phase transition in the vicinity of the nanophotonic structures
can perturb the resonance and consequently the emission response.
In this work, we use the surface plasmon resonance in Aluminum gratings to achieve the narrow
spectral response. The plasmonic structures can effectively couple the surface plasmon
polaritons at the metal-air interface to radiative thermal emission [193, 194]. A phase change
material, Vanadium Dioxide (VO2) was used to dynamically disrupt the excitation of surface
plasmon polaritons and switch off the emission response on demand. We demonstrate two
implementations of emission switching. In one approach VO2 was used to turn off the grating
diffraction and thus perturb the energy momentum matching condition to excite the surface
plasmon polaritons. The second approach used VO2 to screen the grating structure from
incident radiation to prevent the excitation of surface plasmon polaritons. The ability to
modulate the emissivity of materials on demand can find numerous applications, including
chemical analysis, biosensing and environmental monitoring, etc. We had studied our
switchable gratings in a reduced dimensional frame work using the intrinsic decay rates of the
system to arrive at design rules to realize narrow line width with maximum absorption.
8.2 Theory
The emissivity of thermal emitters, made of reciprocal materials, is equal to its absorptivity for
a given frequency, direction, and polarization as dictated by the Kirchhoff’s law of thermal
76
radiation [147, 195]. To realize a narrowband thermal emission spectrum, the absorptivity of
the emitter has to be maximized at a target wavelength while suppressing it at every other
wavelengths [189]. This can be made possible by pattering the materials with feature sizes
comparable to the wavelength of light. The wave interference effects in resonant microphotonic
structures can tailor the spectral response of underlying materials [188]. Coupled mode theory
is a useful tool to analyze resonant absorption in microphotonic structures and derive design
rules to tune the thermal emission [196]. For a single-mode resonator coupling to externally
incident wave, its spectral absorption coefficient () A has a Lorentzian line shape [188]:
22
0
4
()
( ) ( )
rad abs
rad abs
A
=
− + +
(8.1)
where 𝜔 0
is the resonant frequency, Γ
𝑟𝑎𝑑 is the external radiative leakage rate of the resonance
coming from the coupling of the resonance to the output port, and Γ
𝑎𝑏𝑠 is the intrinsic loss rate
of the resonance due to material absorption. At resonance (𝜔 = 𝜔 0
), the absorption can achieve
the maximum value of 1 by matching the external leakage and intrinsic loss rates:
rad abs
= (8.2)
The matching condition is called critical coupling [188]. The concept of critical coupling plays
a significant role in understanding the absorption and emission properties of resonant
microstructures [147, 196]. Even for materials with very low intrinsic absorption, the resonant
absorption can reach 100% by satisfying the critical coupling condition [188]. The quality
factor of the resonances depends on both Γ
𝑟𝑎𝑑 and Γ
𝑎𝑏𝑠 through the expression:
0
rad abs
Q
=
+
(8.3)
77
Narrower resonances have higher quality factor. For a resonance at frequency 𝜔 0
, the two
parameters Γ
𝑟𝑎𝑑 and Γ
𝑎𝑏𝑠 thus completely determine the height and width of the absorption
peak.
We can thus represent the performance of any structure within a reduced-dimensionality
parameter space defined by the radiative and absorptive rates. This is pictured in Fig. 8.1. The
peak absorption is given by the background color map. The red diagonal, dashed line indicates
the critical coupling condition, for which Γ
𝑟𝑎𝑑 = Γ
𝑎𝑏𝑠 , and the peak absorption is equal to 1.
Away from this line, the peak absorption decreases. The black, dashed lines indicate contours
along which Q is constant. Q increases toward the bottom left of the graph. From the graph,
we can see that for the tallest, narrowest emission peak, it is desirable to have small, equal
values of Γ
𝑟𝑎𝑑 = Γ
𝑎𝑏𝑠 . Several example spectra are illustrated on the graph. For case A, Γ
𝑟 𝑎 𝑑 =
Γ
𝑎𝑏𝑠 = 0.94. The spectrum is relatively broad (Q=100), with a peak value of 1. For case B,
Γ
𝑟𝑎𝑑 = 1.79 and Γ
𝑟𝑎𝑑 = 0.09 (equi-Q line). These values were chosen to set Q = 100, as in
case A. The spectral width is thus the same, but the peak absorption is much lower. For case
C, Γ
𝑟𝑎𝑑 = Γ
𝑎𝑏𝑠 = 0.09 , making Q = 1000. Since this case falls on the critical coupling line,
the peak absorption is again 1, with a much narrower spectrum than for case A.
78
Figure 8.1 Mapping resonances to Γ
𝑟𝑎𝑑 − Γ
𝑎𝑏𝑠 parameter space. Resonance A has a quality factor of 100 and
peak absorption of 1. Resonance B has a quality factor of 100 and peak absorption of 0.19. Resonance C has
a quality factor of 1000 and peak absorption of 1
Below, we present several approaches to design switchable microphotonic structures with tall,
narrow emission peaks. The performance of any particular structure can be represented within
a reduced dimensional space determined by the decay parameters. We visualize the effect of
varying the structural dimensions of any given design within the 2D Γ
𝑟𝑎𝑑 − Γ
𝑎𝑏𝑠 parameter
space, arriving at design rules for achieving the desired emissive performance.
8.3 Results and Discussion
We first consider a bare aluminum grating on a silica substrate, as shown schematically in Fig.
8.2(a). An initial set of parameters were chosen to obtain an Al-air surface plasmon mode in
the infrared range. We set the lattice constant a = 10µm, the air gap width w = 1µm, the grating
height h = 0.4µm, and the Al thickness t = 1µm. We simulated the absorption spectrum and
79
electric field distribution for normally-incident light with polarization perpendicular to the
grating, using Lumerical FDTD solutions.
The electric field intensity profile in the grating at resonance is shown in Fig. 8.2(b), indicating
the excitation of a surface plasmon resonance at the Aluminum-air boundary. The intensity at
the interface is enhanced by a factor of 450 relative to the incident plane wave. The absorption
spectrum, shown in Fig. 8.2(c), has a narrow peak at 10µm with a quality factor of ~1469. The
wavelength of the peak can be tuned by varying the lattice constant, as shown in Fig. 8.2(c).
Table 8.1 lists the wavelength and quality factor of the absorption peak as a varies from 8µm
to 12 µm. The resonance red shifts with increasing lattice constant, with a value close to the
grating periodicity. The peak absorption decreases with increasing lattice constant, while the
quality factor increases.
80
Figure 8.2 Aluminum grating on glass substrate. a) Schematic. b) Electric field intensity (|E|
2
) of grating with
w = 1µm, h = 0.4µm, t = 1µm, and a = 10µm. c) Absorption of Aluminum-air grating with varying
periodicity for normally incident light polarized perpendicular to the grating
Table 8.1. Wavelength and quality factor of absorption peaks for aluminum grating
a (µm) 8 9 10 11 12
λ (µm) 8.03 9.02 10.01 11.01 12.01
Q 651 1015 1469 2081 2756
One strategy for creating a structure with tunable absorption is to introduce VO2 into the grating
grooves, as shown in Fig. 8.3(a). When the VO2 is in the insulating state, we expect the structure
to act as a metallic grating (Fig. 8.3(b)). When the VO2 changes to the metallic state, however,
we expect the grating to effectively “disappear,” making the structure one continuous metal
layer (Fig. 8.3(c)). Since the grating structure provides the necessary momentum-matching
criterion for exciting the surface-plasmon mode, we expect the disappearance of the grating to
eliminate the absorption peak.
81
Figure 8.3 a) Aluminum grating on glass substrate with VO 2-filled grooves. The grating structure with b)
insulating and c) metallic VO 2. Absorptivity of grating with d) insulating and e) metallic VO 2 for varying
lattice constants. Structure dimensions: w = 1 µm, h = 0.4 µm, t = 1 µm. f-g) Electric field intensity
distribution corresponding to resonance at 10µm for f) insulating and g) metallic state of VO 2
The absorption spectra of the gratings are shown in Fig. 8.3(d) and 8.3(e) for the insulating and
metallic states of VO2, respectively. For any given grating periodicity, the spectrum exhibits a
narrow spectral feature when VO2 is in the insulating state, similar to the bare aluminum
gratings of Fig. 8.2(c). The absorption feature is turned off by the phase transition; the
absorption is low and nearly featureless. As for the bare aluminum gratings, increasing the
lattice constant shifts the resonances to higher wavelength and reduces the peak absorption.
82
The introduction of VO2 also introduces an additional mode at lower wavelengths, as seen in
Fig. 8.3(d). The lower-wavelength mode is broader and has a much smaller absorption peak.
Fig. 8.3(f) and 8.3(g) show the intensity profiles for the insulating and metallic states,
respectively, for the resonant wavelength. The intensity profile in the insulating state has the
character of a surface plasmon, while the intensity profile in the metallic state arises from a
reflected plane wave.
To obtain the tallest, narrowest, switchable absorption feature in our gratings, we analyze our
results using coupled mode theory. In particular, we study the effect of grating width and height
on the intrinsic absorptive and radiative decay rates of the dominant (higher wavelength) mode.
The background color map in Fig. 8.4 shows the peak absorption as a function of the decay
rates. The diagonal white region marks the critical coupling condition, 𝛤 𝑎𝑏𝑠 = 𝛤 𝑟𝑎𝑑 , at which a
peak absorptivity of 1 is achieved. The dashed-line contours on the plot indicate constant values
of the quality factor Q. To achieve a tall, narrow peak in the on state, we want to design gratings
that lie along the white region, as close to the bottom left of the plot as possible.
83
Figure 8.4 Peak absorption (background color map) and quality factor (dashed contour lines) as a function of
the absorptive and radiative decay rates, for various choices of grating parameters w and h for the grating
geometry of Fig. 8.2(a).
We systematically vary the grating width and height and fit the calculated absorption spectrum
to Eq. 8.1 to extract Γabs and Γrad for the insulating state of VO2. The results are shown as
symbols in Fig. 8.4. Several main trends are apparent.
First, with a decrease in grating height, both decay rates decrease, and the quality factor
increases. See, for example, the blue symbols corresponding to w =500nm. As the grating
height is decreased from h = 500nm (blue triangle) to 300nm (blue circle), both 𝛤 𝑎𝑏𝑠 and 𝛤 𝑟𝑎𝑑
decrease. A similar trend with grating height is apparent for other choices of w (red, yellow,
purple, and green curves). Decreasing the grating height thus increases the quality factor of the
resonance. However, the curves shift away from the diagonal as the grating height decreases,
reducing the peak absorption value in the “on” (insulating) state.
84
Second, increasing the slot width increases the peak absorptivity. This trend can be observed
by tracing the path of the colored circles from blue (w = 500nm) to red, yellow, purple, and
green (w = 1500nm); the green circle lies closest to the white, diagonal region. However, this
increase in peak absorptivity comes at the cost of decreasing quality factor, as both Γabs and
Γrad increase.
A second strategy for creating a tunable grating is to use VO2 as a switchable screen. The design
is shown schematically in Fig. 8.5(a). We fill the grating grooves with an IR transparent
material (ZnS) and cover the structure with a thin layer of VO 2 (thickness tf = 100nm). At low
temperature, VO2 is insulating, supporting the existence of a surface plasmon mode on the
grating. The mode can be seen in the field profile of Fig. 8.5(b). In the metallic state, the VO2
acts as a reflective screen, preventing normally-incident light from “seeing” the grating (Fig.
8.5(c)). Since the incident light can no longer excite a surface plasmon mode, the absorption
peaks visible in the insulating state (Fig. 8.5(d)) disappear for the metallic state (Fig. 8.5(e)).
85
Figure 8.5 a) Aluminum-ZnS grating on glass substrate with a VO 2 top layer. b) Electric field intensity (|E|
2
)
in the insulating state of VO 2. c) Electric field intensity (|E|
2
) in the metallic state of VO 2. d) Absorption of
grating with insulating VO 2 for varying lattice constants. e) Absorptivity of grating with metallic VO 2 for
varying lattice constants. Structure dimensions: w = 1 µm, h = 0.4 µm, t = 1 µm, t f = 100nm.
In addition to the dominant, higher-wavelength mode, a second mode is visible in the spectrum
(Fig. 8.5(a)). The lower-wavelength mode is broader and has a lower peak height. As the
grating period is increased, both resonances shift to the right, and their separation increases.
Inspection of metallic state spectrum (Fig. 8.5(e)) reveals that there is still some residual
absorption in the “off” state. To minimize the residual absorption, we reduced the amount of
VO2 material present in the structure, as in Fig. 8.6(a). Rather than using a continuous VO2
layer as a screen, we use a finite-width shunt to cover the grating groove. From Fig. 8.6(b) and
8.6(d), we see that in the insulating state of VO2, the structure behaves as a grating and exhibits
86
a narrow-linewidth absorption feature. As the shunt width and thickness reduced, the linewidth
of the feature reduces, while the peak height remains nearly constant. In the metallic state (Fig.
8.6(c) and 8.6(e)), absorptive feature disappears, and the residual absorption is decreased
relative to the continuous VO2 layer design of Fig. 8.5(e).
Figure 8.6 a) Aluminum-ZnS grating on Silica substrate with VO 2 shunts. b,c) Absorption of grating with b)
insulating and c) metallic VO 2 for varying shunt width (a = 10µm, w = 1µm, h = 0.4µm, t = 1µm, t s =
100nm). d,e) ) Absorption of grating with b) insulating and c) metallic VO 2 for varying shunt thickness (a =
10µm, w = 1µm, h = 0.4µm, t = 1µm, w s = 1.5w).
Figure 8.7 shows the effect of shunt size on the absorptive ( Γabs) and radiative decay ( Γrad) rates
of the dominant mode. For a given grating periodicity, reducing the shunt thickness shifts the
decay rates downward and to the left, along the diagonal. The absorptive and radiative decay
87
rates scale almost equally with shunt thickness, increasing the quality factor while maintaining
near-maximum peak absorption. In the limit of zero shunt thickness (ts = 0), the quality factor
approaches the quality factor of the underlying Al/ZnS grating. Increasing the shunt
coverage(ws) reduces the quality factor and shifts the curves radially As the width of the film
increases the quality factor of the resonances decreases. But the peak absorption remains almost
the same. The shunt design thus provides a method for introducing absorption switching, while
(1) keeping the linewidth and absorption peak height as close as possible to the underlying,
VO2-free grating, and (2) providing low residual absorption in the off state.
88
Figure 8.7 Peak absorption (background color map) and quality factor (dashed contour lines) as a function of
the absorptive and radiative decay rates, with varying shunt thickness(t s) and width (w s) for a grating
periodicity a = 10µm. The red curve is for a VO 2 shunt covers only the grating gaps and blue curve is for a
continuous VO 2 film
8.4 Conclusion
In conclusion, we had proposed a switchable system with tunable narrowband
absorption/emission. The emissivity modulation is based on the switching off a surface
plasmon mode in metallic grating using VO2 phase change material. We put forward two design
architectures to achieve the switching. In the first approach, the surface plamon resonance was
turned off by making the gratings disappear using VO2 fillings. In the second approach the
gratings are screened from incoming radiation using VO2 layer coated on the top. We have
analyzed the effect of structural parameters on the emission using coupled mode theory and
provided useful insights in designing gratings with desired spectral emission properties
89
Conclusions and Outlook
9.1 Conclusion
We had proposed a mechanism to dynamically switch the emissivity of a plasmonic
nanostructures. The narrow band emission spectrum was dynamically turned on and off using
a phase change material, VO2. The ability to modulate the emissivity of materials on demand
can find numerous applications, including chemical analysis, biosensing and environmental
monitoring, etc. We had studied our switchable gratings in a reduced dimensional frame work
using the intrinsic decay rates of the system to arrive at design rules that dictates the realization
of narrow line width with maximum absorption.
9.2 Outlook
Switchable emission in nanophotonic structures can ultimately lead to the development of
materials that can self-regulate their temperature. The ability to control the temperature rise
due to light absorption in Nano patterned materials is of fundamental interest. This can provide
a powerful tool for the manipulation of nanoscale thermally activated processes. For linear
materials the steady state temperature, increases linearly with the incident optical power. Thus,
90
the temperature can be maintained to desired levels by maintaining constant optical inputs to
absorbing materials.
The method works well for most of the common applications. But one drawback to this
approach is that the temperature is highly sensitive to the optical power. Any spatial or temporal
fluctuation in the optical power will affect the temperature and its distribution. There are
occasions when we need to worry about the stability of temperature rather than the maximum
attainable value. In such cases the temperature can be maintained spatially by locally adjusting
the source power accounting for the spatial variation in delivered power. The temporal
fluctuations can be adjusted by scaling the power up or down accordingly. The temperature
sensing and adjustments must be made instantaneously and to extreme spatial resolutions to
prevent local heating. The issue become more complicated and almost impossible to tackle
when the power fluctuates both in time and space, owing to the delay in sensing and adjusting
the source power.
In well controlled laboratory frame work, this doesn’t seem to be a problem. Thanks to the
highly stable power sources and well-maintained environmental variables. Once we try to
control the optical heating in a remote object in real environment, the above-mentioned power
fluctuations become a significant problem. The atmosphere perturbs even the highest quality
laser beams to a large extend. The perturbations vary time to time and can become totally
unpredictable. The issue is well known among astronomers who use adaptive optics and guide-
star corrections as a way around. These solutions are impractical when we try to
instantaneously compensate for the fluctuations. The key to solve the problem lies in intelligent
materials which can detect the change in temperature and adjust themselves. Systems designed
91
to self-correct the absorption and thus the temperature can act as optical thermoregulators
which work like the voltage regulators in electrical circuits. Fig. 9.1 shows the schematic
illustration of the working principle. To uniformly heat the device a beam with much bigger
size is used as shown in Fig. 9.1a. The part of the beam which heats up the device is shown as
shaded grey region in Fig. 9.1b. The ideal beam has a Gaussian intensity profile as shown in
Figure 1b. But the intensity profile will get distorted by the atmospheric perturbations as shown
by the red curve in Fig. 9.1b. The range of variations of intensities is shown as shaded orange
region. Figure 9.1c shows the absorbed power as a function of the input power for the case of
a linear device (broken black curve) and nonlinear device (solid green curve). The temperature
rises along the linear device for the ideal (broken blue) and distorted (broken red) beam is given
in Fig. 9.1d. The temperature varies uniformly across the device due to the ideal beam and non-
uniformly due to the distorted beam. On the other hand, the nonlinear device maintains the
temperature across the device surface regardless of the power fluctuations across it.
92
Figure 9.1 Remote heating: Problem and Proposed Solution. a) the schematic of the device illuminated with
laser beam. b) Intensities of ideal and distorted Gaussian beam across the device (shaded grey region). The
range of power fluctuations is shown in shaded orange. c) Absorbed power vs incident power for linear
(black) and nonlinear (green) device. d) Temperature profile across the linear device (blue and red) and
nonlinear device (green)
It is possible to design and fabricate a self-regulating device capable of maintaining steady
temperature under a range of laser power perturbations. The proposed device has a
nanopatterned metallic structures coated with a phase change material. The working principle
of the device is illustrated in Fig. 9.2.
93
Figure 9.2 a) The schematic of the optical thermoregulatory. b) The shift in resonance with feedback signal c)
The switching between states and temperature oscillations. d) Pinning down of temperature in the device
The nanostructured device has an absorptive plasmon resonance mode in the near infrared
range, as shown in Fig. 9.2b, which is highly sensitive to the refractive index of the surrounding
medium. The phase change material acts as a temperature sensor and absorption switch. At the
operating wavelength, λ0, the system has a high absorption state |1⟩ at low temperature and low
absorption state |2⟩ at high temperature. Under laser illumination, the device heats up and
temperature increases in the material. As the temperature rises, the surrounding medium
undergo phase transition which shifts the resonance. The absorption switches to state |2⟩ and
goes down, so is the temperature. When the temperature goes down below the phase transition,
94
the medium again switches back to state|1⟩, which brings the temperature back to high value.
For a range of input powers, the absorption switches back and forth between high and low
values and temperature pin down between the phase transition temperatures of the surrounding
medium. The absorption of the device is coupled to its temperature through phase change
material in the surrounding medium.
95
References
1. Marago, O.M., et al., Optical trapping and manipulation of nanostructures. Nat Nano,
2013. 8(11): p. 807-819.
2. Grier, D.G., A revolution in optical manipulation. Nature, 2003. 424(6950): p. 810-816.
3. Gao, D., et al., Optical manipulation from the microscale to the nanoscale: fundamentals,
advances and prospects. Light: Science & Applications, 2017. 6(9): p. e17039-e17039.
4. Ashkin, A., et al., Observation of a single-beam gradient force optical trap for dielectric
particles. Optics Letters, 1986. 11(5): p. 288-290.
5. Ashkin, A., Optical trapping and manipulation of neutral particles using lasers.
Proceedings of the National Academy of Sciences, 1997. 94(10): p. 4853.
6. MacDonald, M.P., G.C. Spalding, and K. Dholakia, Microfluidic sorting in an optical
lattice. Nature, 2003. 426(6965): p. 421-424.
7. Wang, M.M., et al., Microfluidic sorting of mammalian cells by optical force switching.
Nature Biotechnology, 2005. 23(1): p. 83-87.
8. Jonáš, A. and P. Zemánek, Light at work: The use of optical forces for particle
manipulation, sorting, and analysis. ELECTROPHORESIS, 2008. 29(24): p. 4813-4851.
9. Brzobohatý, O., et al., Experimental demonstration of optical transport, sorting and self-
arrangement using a ‘tractor beam’. Nature Photonics, 2013. 7: p. 123.
10. Yang, A.H.J., T. Lerdsuchatawanich, and D. Erickson, Forces and Transport Velocities for
a Particle in a Slot Waveguide. Nano Letters, 2009. 9(3): p. 1182-1188.
11. Dholakia, K., P. Reece, and M. Gu, Optical micromanipulation. Chemical Society Reviews,
2008. 37(1): p. 42-55.
12. Moffitt, J.R., et al., Recent Advances in Optical Tweezers. Annual Review of Biochemistry,
2008. 77(1): p. 205-228.
96
13. Ashkin, A., Trapping of Atoms by Resonance Radiation Pressure. Physical Review Letters,
1978. 40(12): p. 729-732.
14. Ashkin, A. and J.P . Gordon, Cooling and trapping of atoms by resonance radiation
pressure. Optics Letters, 1979. 4(6): p. 161-163.
15. Zobay, O. and B.M. Garraway, Two-Dimensional Atom Trapping in Field-Induced
Adiabatic Potentials. Physical Review Letters, 2001. 86(7): p. 1195-1198.
16. Ashkin, A. and J.M. Dziedzic, Optical trapping and manipulation of viruses and bacteria.
Science, 1987. 235(4795): p. 1517.
17. Ashkin, A., J.M. Dziedzic, and T. Yamane, Optical trapping and manipulation of single
cells using infrared laser beams. Nature, 1987. 33 0(6150): p. 769-771.
18. Pang, Y . and R. Gordon, Optical Trapping of a Single Protein. Nano Letters, 2012. 12(1):
p. 402-406.
19. Wang, M.D., et al., Stretching DNA with optical tweezers. Biophysical Journal, 1997. 72(3):
p. 1335-1346.
20. Chiu, D.T. and R.N. Zare, Biased Diffusion, Optical Trapping, and Manipulation of Single
Molecules in Solution. Journal of the American Chemical Society, 1996. 1 18(27): p. 6512-
6513.
21. Hummon, M.T., et al., 2D Magneto-Optical Trapping of Diatomic Molecules. Physical
Review Letters, 2013. 1 10(14): p. 143001.
22. Arai, Y., et al., Tying a molecular knot with optical tweezers. Nature, 1999. 399(6735): p.
446-448.
23. Atwater, H.A., et al., Materials challenges for the Starshot lightsail. Nature Materials,
2018. 17(10): p. 861-867.
24. Ilic, O. and H.A. Atwater, Self-stabilizing photonic levitation and propulsion of
nanostructured macroscopic objects. Nature Photonics, 2019. 13(4): p. 289-295.
25. Michaelis, M.M. and A. Forbes, Laser propulsion : a review : review article. South African
Journal of Science, 2006. 102(7-8): p. 289-295.
26. UCSB&NASA. Starlight. 2009; Available from:
https://www.deepspace.ucsb.edu/projects/starlight.
27. breakthroughstarshot. breakthroughinitiatives. 2019; Available from:
https://breakthroughinitiatives.org/initiative/3.
97
28. Anglada-Escudé, G., et al., A terrestrial planet candidate in a temperate orbit around
Proxima Centauri. Nature, 2016. 536: p. 437.
29. Lubin, P. A Roadmap to Interstellar Flight. 2016; Available from:
http://arxiv.org/abs/1604.01356.
30. JAXA. IKAROS. 2010; Available from:
https://global.jaxa.jp/countdown/f17/overview/ikaros_e.html.
31. NASA. Nanosail-D 2010; Available from:
https://www.nasa.gov/mission_pages/smallsats/nanosaild.html.
32. PlanetarySociety. LightSail. 2015; Available from:
http://www.planetary.org/explore/projects/lightsail-solar-sailing/.
33. Ashkin, A., History of optical trapping and manipulation of small-neutral particle, atoms,
and molecules. IEEE Journal of Selected Topics in Quantum Electronics, 2000. 6(6): p.
841-856.
34. Born, M., E. Wolf, and A.B. Bhatia, Principles of Optics: Electromagnetic Theory of
Propagation, Interference and Diffraction of Light2000: Cambridge University Press.
35. Erickson, D., et al., Nanomanipulation using near field photonics. Lab on a Chip, 2011.
11(6): p. 995-1009.
36. Noda, S. and T. Baba, Roadmap on Photonic Crystals2013: Springer US.
37. Joannopoulos, J.D., et al., Photonic Crystals: Molding the Flow of Light - Second
Edition2011: Princeton University Press.
38. Huang, N., et al., Optical Epitaxial Growth of Gold Nanoparticle Arrays. Nano Letters,
2015. 15(9): p. 5841-5845.
39. Jaquay, E., et al., Light-Assisted, Templated Self-Assembly of Gold Nanoparticle Chains.
Nano Letters, 2014. 14(9): p. 5184-5188.
40. Krishnan, A., et al., Enhanced and selective optical trapping in a slot-graphite photonic
crystal. Optics Express, 2016. 24(20): p. 23271-23279.
41. Wu, S.-H., et al., Near-Field, On-Chip Optical Brownian Ratchets. Nano Letters, 2016.
16(8): p. 5261-5266.
42. Johnson, S.G., et al., Guided modes in photonic crystal slabs. Physical Review B, 1999.
60(8): p. 5751-5758.
98
43. Fan, S. and J.D. Joannopoulos, Analysis of guided resonances in photonic crystal slabs.
Physical Review B, 2002. 65(23): p. 235112.
44. Ochiai, T. and K. Sakoda, Dispersion relation and optical transmittance of a hexagonal
photonic crystal slab. Physical Review B, 2001. 63(12): p. 125107.
45. Crozier, K.B., et al., Air-bridged photonic crystal slabs at visible and near-infrared
wavelengths. Physical Review B, 2006. 73(11): p. 115126.
46. Mansuripur, M., Radiation pressure and the linear momentum of the electromagnetic field.
Optics Express, 2004. 12(22): p. 5375-5401.
47. Bradshaw, D.S. and D.L. Andrews, Manipulating particles with light: radiation and
gradient forces. European Journal of Physics, 2017. 38(3): p. 034008.
48. Moura, J.P., et al., Centimeter-scale suspended photonic crystal mirrors. Optics Express,
2018. 26(2): p. 1895-1909.
49. Chen, X., et al., High-finesse Fabry–Perot cavities with bidimensional Si3N4 photonic-
crystal slabs. Light: Science & Applications, 2017. 6(1): p. e16190-e16190.
50. Dholakia, K. and T. Cizmar, Shaping the future of manipulation. Nat Photon, 2011. 5(6): p.
335-342.
51. Padgett, M. and R. Bowman, Tweezers with a twist. Nat Photon, 2011. 5(6): p. 343-348.
52. Lin, S., et al., Surface-Enhanced Raman Scattering with Ag Nanoparticles Optically
Trapped by a Photonic Crystal Cavity. Nano Letters, 2013. 13(2): p. 559-563.
53. Soltani, M., et al., Nanophotonic trapping for precise manipulation of biomolecular arrays.
Nat Nano, 2014. 9(6): p. 448-452.
54. Jannasch, A., et al., Nanonewton optical force trap employing anti-reflection coated, high-
refractive-index titania microspheres. Nat Photon, 2012. 6(7): p. 469-473.
55. Franosch, T., et al., Resonances arising from hydrodynamic memory in Brownian motion.
Nature, 2011. 478(7367): p. 85-88.
56. Jannasch, A., M. Mahamdeh, and E. Schäffer, Inertial Effects of a Small Brownian Particle
Cause a Colored Power Spectral Density of Thermal Noise. Physical Review Letters, 2011.
107(22): p. 228301.
57. Dong, J., et al., Optical trapping with high forces reveals unexpected behaviors of prion
fibrils. Nat Struct Mol Biol, 2010. 17(12): p. 1422-1430.
99
58. Fazal, F.M. and S.M. Block, Optical tweezers study life under tension. Nat Photon, 2011.
5(6): p. 318-321.
59. Bormuth, V ., et al., Optical trapping of coated microspheres. Optics Express, 2008. 16(18):
p. 13831-13844.
60. Selhuber-Unkel, C., et al., Quantitative Optical Trapping of Single Gold Nanorods. Nano
Letters, 2008. 8(9): p. 2998-3003.
61. Messina, E., et al., Plasmon-Enhanced Optical Trapping of Gold Nanoaggregates with
Selected Optical Properties. ACS Nano, 2011. 5(2): p. 905-913.
62. Pelton, M., et al., Optical trapping and alignment of single gold nanorods by using plasmon
resonances. Optics Letters, 2006. 31(13): p. 2075-2077.
63. Toussaint, K.C., et al., Plasmon resonance-based optical trapping of single and multiple
Au nanoparticles. Optics Express, 2007. 15(19): p. 12017-12029.
64. Grigorenko, A.N., et al., Nanometric optical tweezers based on nanostructured substrates.
Nat Photon, 2008. 2(6): p. 365-370.
65. Juan, M.L., et al., Self-induced back-action optical trapping of dielectric nanoparticles.
Nat Phys, 2009. 5(12): p. 915-919.
66. Juan, M.L., M. Righini, and R. Quidant, Plasmon nano-optical tweezers. Nat Photon, 2011.
5(6): p. 349-356.
67. Lin, S., E. Schonbrun, and K. Crozier, Optical Manipulation with Planar Silicon Microring
Resonators. Nano Letters, 2010. 10(7): p. 2408-2411.
68. Mandal, S., X. Serey, and D. Erickson, Nanomanipulation Using Silicon Photonic Crystal
Resonators. Nano Letters, 2010. 10(1): p. 99-104.
69. Jaquay, E., et al., Light-Assisted, Templated Self-Assembly Using a Photonic-Crystal Slab.
Nano Letters, 2013. 13(5): p. 2290-2294.
70. Berthelot, J., et al., Three-dimensional manipulation with scanning near-field optical
nanotweezers. Nat Nano, 2014. 9(4): p. 295-299.
71. Jing, P., J. Wu, and L.Y. Lin, Patterned Optical Trapping with Two-Dimensional Photonic
Crystals. ACS Photonics, 2014. 1(5): p. 398-402.
72. Milord, L., et al., Engineering of slow Bloch modes for optical trapping. Applied Physics
Letters, 2015. 106(12): p. 121110.
100
73. Ma, J., L.J. Martínez, and M.L. Povinelli, Optical trapping via guided resonance modes in
a Slot-Suzuki-phase photonic crystal lattice. Optics Express, 2012. 20(6): p. 6816-6824.
74. Almeida, V.R., et al., Guiding and confining light in void nanostructure. Optics Letters,
2004. 29(11): p. 1209-1211.
75. Martínez, L.J., et al., Design and optical characterization of high-Q guided-resonance
modes in the slot-graphite photonic crystal lattice. Optics Express, 2013. 21(25): p. 30975-
30983.
76. Jackson, J.D., Classical electrodynamics1999: Third edition. New York : Wiley, [1999]
©1999.
77. Huang, N., L.J. Martínez, and M.L. Povinelli, Tuning the transmission lineshape of a
photonic crystal slab guided-resonance mode by polarization control. Optics Express,
2013. 21(18): p. 20675-20682.
78. Serey, X., et al., DNA Transport and Delivery in Thermal Gradients near Optofluidic
Resonators. Physical Review Letters, 2012. 108(4): p. 048102.
79. Kedenburg, S., et al., Linear refractive index and absorption measurements of nonlinear
optical liquids in the visible and near-infrared spectral region. Optical Materials Express,
2012. 2(11): p. 1588-1611.
80. Neuman, K.C. and S.M. Block, Optical trapping. Review of Scientific Instruments, 2004.
75(9): p. 2787-2809.
81. Parthasarathy, R., Rapid, accurate particle tracking by calculation of radial symmetry
centers. Nat Meth, 2012. 9(7): p. 724-726.
82. Wong, W.P. and K. Halvorsen, The effect of integration time on fluctuation measurements:
calibrating an optical trap in the presence of motion blur . Optics Express, 2006. 14(25): p.
12517-12531.
83. Hosokawa, M., et al., Size-Selective Microcavity Array for Rapid and Efficient Detection
of Circulating Tumor Cells. Analytical Chemistry, 2010. 82(15): p. 6629-6635.
84. Hur, S.C., A.J. Mach, and D. Di Carlo, High-throughput size-based rare cell enrichment
using microscale vortices. Biomicrofluidics, 2011. 5(2): p. 022206.
85. Preinerstorfer, B., M. Lämmerhofer, and W. Lindner, Synthesis and application of novel
phenylboronate affinity materials based on organic polymer particles for selective trapping
of glycoproteins. Journal of Separation Science, 2009. 32(10): p. 1673-1685.
101
86. Wiklund, M., S. Nilsson, and H.M. Hertz, Ultrasonic trapping in capillaries for trace-
amount biomedical analysis. Journal of Applied Physics, 2001. 90(1): p. 421-426.
87. Agarwal, G. and C. Livermore, Chip-based size-selective sorting of biological cells using
high frequency acoustic excitation. Lab on a Chip, 2011. 11(13): p. 2204-2211.
88. Kim, J., et al., A high-efficiency microfluidic device for size-selective trapping and sorting.
Lab on a Chip, 2014. 14(14): p. 2480-2490.
89. Petit, T., et al., Selective Trapping and Manipulation of Microscale Objects Using Mobile
Microvortices. Nano Letters, 2012. 12(1): p. 156-160.
90. Rogers, P. and A. Neild, Selective particle trapping using an oscillating microbubble. Lab
on a Chip, 2011. 11(21): p. 3710-3715.
91. Ricárdez-Vargas, I., et al., Modulated optical sieve for sorting of polydisperse
microparticles. Applied Physics Letters, 2006. 88(12): p. 121116.
92. Righini, M., et al., Parallel and selective trapping in a patterned plasmonic landscape. Nat
Phys, 2007. 3(7): p. 477-480.
93. Chaumet, P.C. and A. Rahmani, Optical tweezers: Dressed for success. Nat Nano, 2014.
9(4): p. 252-253.
94. Krishnan, A., S.-H. Wu, and M. Povinelli, Tunable size selectivity and nanoparticle
immobilization on a photonic crystal optical trap. Optics Letters, 2018. 43(21): p. 5399-
5402.
95. Dholakia, K. and T. Čižmár, Shaping the future of manipulation. Nature Photonics, 2011.
5: p. 335.
96. Maragò, O.M., et al., Optical trapping and manipulation of nanostructures. Nature
Nanotechnology, 2013. 8: p. 807.
97. Juan, M.L., et al., Self-induced back-action optical trapping of dielectric nanoparticles.
Nature Physics, 2009. 5: p. 915.
98. Yang, A.H.J., et al., Optical manipulation of nanoparticles and biomolecules in sub-
wavelength slot waveguides. Nature, 2009. 457(7225): p. 71-75.
99. Soltani, M., et al., Nanophotonic trapping for precise manipulation of biomolecular arrays.
Nature Nanotechnology, 2014. 9: p. 448.
100. Braun, D. and A. Libchaber, Trapping of DNA by Thermophoretic Depletion and
Convection. Physical Review Letters, 2002. 89(18): p. 188103.
102
101. Yu, L.-H. and Y.-F. Chen, Concentration-Dependent Thermophoretic Accumulation for
the Detection of DNA Using DNA-Functionalized Nanoparticles. Analytical Chemistry,
2015. 87(5): p. 2845-2851.
102. Wienken, C.J., et al., Protein-binding assays in biological liquids using microscale
thermophoresis. Nature Communications, 2010. 1: p. 100.
103. Chen, J., et al., Thermal gradient induced tweezers for the manipulation of particles
and cells. Scientific Reports, 2016. 6: p. 35814.
104. Duhr, S. and D. Braun, Why molecules move along a temperature gradient. Proceedings
of the National Academy of Sciences, 2006. 103(52): p. 19678-19682.
105. Alois, W., Thermal non-equilibrium transport in colloids. Reports on Progress in
Physics, 2010. 73(12): p. 126601.
106. Ndukaife, J.C., et al., Long-range and rapid transport of individual nano-objects by a
hybrid electrothermoplasmonic nanotweezer. Nature Nanotechnology, 2015. 11: p. 53.
107. Lin, L., et al., Light-Directed Reversible Assembly of Plasmonic Nanoparticles Using
Plasmon-Enhanced Thermophoresis. ACS Nano, 2016. 10(10): p. 9659-9668.
108. Lin, L., et al., Opto-thermophoretic assembly of colloidal matter. Science Advances,
2017. 3(9).
109. Lin, L., et al., Optothermal Manipulations of Colloidal Particles and Living Cells.
Accounts of Chemical Research, 2018. 51(6): p. 1465-1474.
110. Ndukaife, J.C., et al., High-Resolution Large-Ensemble Nanoparticle Trapping with
Multifunctional Thermoplasmonic Nanohole Metasurface. ACS Nano, 2018. 12(6): p.
5376-5384.
111. Peng, X., et al., Optothermophoretic Manipulation of Colloidal Particles in Nonionic
Liquids. The Journal of Physical Chemistry C, 2018.
112. Braibanti, M., D. Vigolo, and R. Piazza, Does Thermophoretic Mobility Depend on
Particle Size? Physical Review Letters, 2008. 100(10): p. 108303.
113. Jiang, H.-R., N. Yoshinaga, and M. Sano, Active Motion of a Janus Particle by Self-
Thermophoresis in a Defocused Laser Beam. Physical Review Letters, 2010. 105(26): p.
268302.
114. Syshchyk, O., et al., Influence of temperature and charge effects on thermophoresis of
polystyrene beads⋆. The European Physical Journal E, 2016. 39(12): p. 129.
103
115. Jamshidi, A., et al., Dynamic manipulation and separation of individual
semiconducting and metallic nanowires. Nature Photonics, 2008. 2: p. 86.
116. Righini, M., et al., Nano-optical Trapping of Rayleigh Particles and Escherichia coli
Bacteria with Resonant Optical Antennas. Nano Letters, 2009. 9(10): p. 3387-3391.
117. van Leest, T. and J. Caro, Cavity-enhanced optical trapping of bacteria using a silicon
photonic crystal. Lab on a Chip, 2013. 13(22): p. 4358-4365.
118. Yang, D., et al., Single nanoparticle trapping based on on-chip nanoslotted nanobeam
cavities. Photonics Research, 2018. 6(2): p. 99-108.
119. Krishnan, A. and M.L. Povinelli, Tunable, polarization-sensitive, dual guided-
resonance modes in photonic crystals. Optics Express, 2019. 27(13): p. 17658-17666.
120. Joannopoulos, J.D., et al., Photonic Crystals: Molding the Flow of Light2008: Princeton
University Press. 304.
121. Koenderink, A.F., A. Alù, and A. Polman, Nanophotonics: Shrinking light-based
technology. Science, 2015. 348(6234): p. 516.
122. Genet, C. and T.W. Ebbesen, Light in tiny holes. Nature, 2007. 445: p. 39.
123. Schuller, J.A., et al., Plasmonics for extreme light concentration and manipulation.
Nature Materials, 2010. 9: p. 193.
124. Burns, M.M., J.-M. Fournier, and J.A. Golovchenko, Optical Matter: Crystallization
and Binding in Intense Optical Fields. Science, 1990. 249(4970): p. 749.
125. Grillet, C., et al., Reconfigurable photonic crystal circuits. Laser & Photonics Reviews,
2010. 4(2): p. 192-204.
126. Bedoya, A.C., et al., Reconfigurable photonic crystal waveguides created by selective
liquid infiltration. Optics Express, 2012. 20(10): p. 11046-11056.
127. Cui, Y ., et al., Dynamic Tuning and Symmetry Lowering of Fano Resonance in
Plasmonic Nanostructure. ACS Nano, 2012. 6(3): p. 2385-2393.
128. Walia, S., et al., Flexible metasurfaces and metamaterials: A review of materials and
fabrication processes at micro- and nano-scales. Applied Physics Reviews, 2015. 2(1): p.
011303.
129. Yu, C.L., et al., Stretchable Photonic Crystal Cavity with Wide Frequency Tunability.
Nano Letters, 2013. 13(1): p. 248-252.
104
130. Mejia, C.A., A. Dutt, and M.L. Povinelli, Light-assisted templated self assembly using
photonic crystal slabs. Optics Express, 2011. 19(12): p. 11422-11428.
131. Magnusson, R., et al., Resonant Photonic Biosensors with Polarization-Based
Multiparametric Discrimination in Each Channel. Sensors, 2011. 11(2).
132. Magnusson, R., The Complete Biosensor. Journal of Biosensors & Bioelectronics, 2013.
4(2).
133. Gao, X., et al., Multiline resonant filters fashioned with different periodic
subwavelength gratings. Optics Letters, 2014. 39(23): p. 6660-6663.
134. Kuo, W.-K. and C.-J. Hsu, Two-dimensional grating guided-mode resonance tunable
filter. Optics Express, 2017. 25(24): p. 29642-29649.
135. Qian, L., et al., Optical notch filter with tunable bandwidth based on guided-mode
resonant polarization-sensitive spectral feature. Optics Express, 2015. 23(14): p. 18300-
18309.
136. Uddin, M.J., T. Khaleque, and R. Magnusson, Guided-mode resonant polarization-
controlled tunable color filters. Optics Express, 2014. 22(10): p. 12307-12315.
137. Wang, Y., et al., Multiband guided-mode resonance filter in bilayer asymmetric metallic
gratings. Optics & Laser Technology, 2018. 103: p. 135-141.
138. Ríos, C., et al., Integrated all-photonic non-volatile multi-level memory. Nature
Photonics, 2015. 9: p. 725.
139. Fan, S., W. Suh, and J.D. Joannopoulos, Temporal coupled-mode theory for the Fano
resonance in optical resonators. JOSA A, 2003. 20(3): p. 569-572.
140. Bravo-Abad, J., et al., Enhanced nonlinear optics in photonic-crystal microcavities.
Optics express, 2007. 15(24): p. 16161-16176.
141. Soljačić, M., et al., Optimal bistable switching in nonlinear photonic crystals. Physical
Review E, 2002. 66(5): p. 055601.
142. Philipp, H.R., Optical Properties of Silicon Nitride. Journal of The Electrochemical
Society, 1973. 120(2): p. 295-300.
143. Krückel, C.J., et al., Linear and nonlinear characterization of low-stress high-
confinement silicon-rich nitride waveguides. Optics Express, 2015. 23(20): p. 25827-
25837.
105
144. starshot, b. breakthroughinitiatives. 2019; Available from:
https://breakthroughinitiatives.org/initiative/3.
145. Atwater, H.A., et al., Materials challenges for the Starshot lightsail. Nature Materials,
2018.
146. Nishihara, T., et al., Ultra-narrow-band near-infrared thermal exciton radiation in
intrinsic one-dimensional semiconductors. Nature Communications, 2018. 9(1): p. 3144.
147. Li, W. and S. Fan, Nanophotonic control of thermal radiation for energy applications
[Invited]. Optics Express, 2018. 26(12): p. 15995-16021.
148. Chan, D.L.C., M. Soljačić, and J.D. Joannopoulos, Thermal emission and design in
one-dimensional periodic metallic photonic crystal slabs. Physical Review E, 2006. 74(1):
p. 016609.
149. Diwekar, M., et al., Midinfrared optical response and thermal emission from plasmonic
lattices on Al films. Physical Review B, 2007. 76(19): p. 195402.
150. Inoue, T., et al., Realization of dynamic thermal emission control. Nature Materials,
2014. 13: p. 928.
151. Planck, M., The Theory of Heat Radiation1912: Dover Publication.
152. De Zoysa, M., et al., Conversion of broadband to narrowband thermal emission
through energy recycling. Nature Photonics, 2012. 6(8): p. 535-539.
153. Noda, S. Thermal emission control by photonic crystals. in 2016 21st OptoElectronics
and Communications Conference (OECC) held jointly with 2016 International Conference
on Photonics in Switching (PS). 2016.
154. Brar, V.W., et al., Electronic modulation of infrared radiation in graphene plasmonic
resonators. Nature Communications, 2015. 6: p. 7032.
155. Liu, X. and W.J. Padilla, Thermochromic Infrared Metamaterials. Advanced Materials,
2016. 28(5): p. 871-875.
156. Qu, Y., et al., Dynamic Thermal Emission Control Based on Ultrathin Plasmonic
Metamaterials Including Phase-Changing Material GST. Laser & Photonics Reviews,
2017. 11(5): p. 1700091.
157. Du, K.-K., et al., Control over emissivity of zero-static-power thermal emitters based
on phase-changing material GST. Light: Science &Amp; Applications, 2017. 6: p. e16194.
106
158. Kocer, H., et al., Thermal tuning of infrared resonant absorbers based on hybrid gold-
VO2 nanostructures. Applied Physics Letters, 2015. 106(16): p. 161104.
159. Sharma, Y ., et al., VO2 based waveguide-mode plasmonic nano-gratings for optical
switching. Optics Express, 2015. 23(5): p. 5822-5849.
160. Savaliya, P.B., et al., Tunable optical switching in the near-infrared spectral regime by
employing plasmonic nanoantennas containing phase change materials. Optics Express,
2017. 25(20): p. 23755-23772.
161. Barnes, W.L., A. Dereux, and T.W. Ebbesen, Surface plasmon subwavelength optics.
Nature, 2003. 424: p. 824.
162. Zhang, J. and L. Zhang, Nanostructures for surface plasmons. Advances in Optics and
Photonics, 2012. 4(2): p. 157-321.
163. Qazilbash, M.M., et al., Mott Transition in VO<sub>2</sub> Revealed by
Infrared Spectroscopy and Nano-Imaging. Science, 2007. 318(5857): p. 1750.
164. Currie, M., M.A. Mastro, and V.D. Wheeler, Characterizing the tunable refractive index
of vanadium dioxide. Optical Materials Express, 2017. 7(5): p. 1697-1707.
165. Zhang, Z., et al., Thermochromic VO2 Thin Films: Solution-Based Processing,
Improved Optical Properties, and Lowered Phase Transformation Temperature. Langmuir,
2010. 26(13): p. 10738-10744.
166. Xu, F., et al., Recent advances in VO2-based thermochromic composites for smart
windows. Journal of Materials Chemistry C, 2018. 6(8): p. 1903-1919.
167. Kats, M.A., et al., Thermal tuning of mid-infrared plasmonic antenna arrays using a
phase change material. Optics Letters, 2013. 38(3): p. 368-370.
168. Miller, K.J., R.F. Haglund, and S.M. Weiss, Optical phase change materials in
integrated silicon photonic devices: review. Optical Materials Express, 2018. 8(8): p. 2415-
2429.
169. Guo, P., et al., Conformal Coating of a Phase Change Material on Ordered Plasmonic
Nanorod Arrays for Broadband All-Optical Switching. ACS Nano, 2017. 11(1): p. 693-701.
170. Wu, S.-H., et al., Thermal homeostasis using microstructured phase-change materials.
Optica, 2017. 4(11): p. 1390-1396.
171. Wu, S.-R., K.-L. Lai, and C.-M. Wang, Passive temperature control based on a phase
change metasurface. Scientific Reports, 2018. 8(1): p. 7684.
107
172. Cortie, M.B., et al., Core-shell nanoparticles with self-regulating plasmonic
functionality. Physical Review B, 2007. 75(11): p. 113405.
173. Muskens, O.L., et al., Antenna-assisted picosecond control of nanoscale phase
transition in vanadium dioxide. Light: Science &Amp; Applications, 2016. 5: p. e16173.
174. Markov, P., et al., Optically Monitored Electrical Switching in VO2. ACS Photonics,
2015. 2(8): p. 1175-1182.
175. Batista, C., R.M. Ribeiro, and V. Teixeira, Synthesis and characterization of VO2-based
thermochromic thin films for energy-efficient windows. Nanoscale Research Letters, 2011.
6(1): p. 301.
176. Cao, J., et al., Strain engineering and one-dimensional organization of metal–insulator
domains in single-crystal vanadium dioxide beams. Nature Nanotechnology, 2009. 4: p.
732.
177. Hodgkinson, J. and R.P. Tatam, Optical gas sensing: a review. Measurement Science
and Technology, 2012. 24(1): p. 012004.
178. Rubio, R., et al., Non-selective NDIR array for gas detection. Sensors and Actuators B:
Chemical, 2007. 127(1): p. 69-73.
179. De Wilde, Y., et al., Thermal radiation scanning tunnelling microscopy. Nature, 2006.
444(7120): p. 740-743.
180. Kittel, A., et al., Near-Field Heat Transfer in a Scanning Thermal Microscope. Physical
Review Letters, 2005. 95(22): p. 224301.
181. Salihoglu, O., et al., Graphene-Based Adaptive Thermal Camouflage. Nano Letters,
2018. 18(7): p. 4541-4548.
182. Ilic, O., et al., Tailoring high-temperature radiation and the resurrection of the
incandescent source. Nature Nanotechnology, 2016. 11: p. 320.
183. Fan, S., Thermal Photonics and Energy Applications. Joule, 2017. 1(2): p. 264-273.
184. Chan, W.R., et al., Toward high-energy-density, high-efficiency, and moderate-
temperature chip-scale thermophotovoltaics. Proceedings of the National Academy of
Sciences, 2013. 1 10(14): p. 5309.
185. Lenert, A., et al., A nanophotonic solar thermophotovoltaic device. Nature
Nanotechnology, 2014. 9: p. 126.
108
186. Rephaeli, E. and S. Fan, Absorber and emitter for solar thermo-photovoltaic systems to
achieve efficiency exceeding the Shockley-Queisser limit. Optics Express, 2009. 17(17): p.
15145-15159.
187. Howell, J.R., M.P. Menguc, and R. Siegel, Thermal Radiation Heat Transfer2015: CRC
Press.
188. Guo, Y . and S. Fan, Narrowband thermal emission from a uniform tungsten surface
critically coupled with a photonic crystal guided resonance. Optics Express, 2016. 24(26):
p. 29896-29907.
189. Inoue, T., et al., Realization of narrowband thermal emission with optical
nanostructures. Optica, 2015. 2(1): p. 27-35.
190. Celanovic, I., D. Perreault, and J. Kassakian, Resonant-cavity enhanced thermal
emission. Physical Review B, 2005. 72(7): p. 075127.
191. Inoue, T., et al., High-Q mid-infrared thermal emitters operating with high power-
utilization efficiency. Optics Express, 2016. 24(13): p. 15101-15109.
192. Pralle, M.U., et al., Photonic crystal enhanced narrow-band infrared emitters. Applied
Physics Letters, 2002. 81(25): p. 4685-4687.
193. Biener, G., et al., Highly coherent thermal emission obtained by plasmonic bandgap
structures. Applied Physics Letters, 2008. 92(8): p. 081913.
194. Ikeda, K., et al., Controlled thermal emission of polarized infrared waves from arrayed
plasmon nanocavities. Applied Physics Letters, 2008. 92(2): p. 021117.
195. Brace, D.B., et al., The laws of radiation and absorption; memoirs by Prévost, Stewart,
Kirchhoff, and Kirchhoff and Bunsen1901, New York; Cincinnati: American Book Co.
196. Zhu, L., et al., Temporal coupled mode theory for thermal emission from a single
thermal emitter supporting either a single mode or an orthogonal set of modes. Applied
Physics Letters, 2013. 102(10): p. 103104.
Abstract (if available)
Abstract
This dissertation work studies the use of resonant light interactions in nanophotonic structures to control the flow of light in two major application areas: Manipulating optical forces and thermal emission. ❧ The Guided resonance in photonic crystals has been extensively used for guiding the light in integrated photonics. The spatial confinement of light resulting from the resonance can be used to generate powerful optical force to manipulate microscopic matter. First, we had experimentally demonstrated the optical trapping of submicron objects various sizer and composition using highly enhanced optical traps in photonic crystals. Applicability of optical trapping tools for nanomanipulation is limited by the available laser power and trap efficiency. The strong confinement of light in a slot-graphite photonic crystal is utilized to develop high-efficiency parallel trapping over a large area. The stiffness is 35 times higher than our previously demonstrated on-chip, near field traps. We demonstrate the ability to trap both dielectric and metallic particles of sub-micron size. ❧ The strong field enhancement leads to secondary light matter interactions via optical heating. We harness residual thermal effects in a low-absorptivity system to manipulate parallel optical trapping of particles on the nanoscale. We show that the size selectivity of the trap can be tuned by adding a non-ionic surfactant to the solution, altering the thermophoretic effect that delivers nanoparticles to trapping sites. We find that the growth kinetics of nanoparticle arrays on the slot-graphite template depends on particle size. This difference is exploited to selectively trap one type of particle out of a binary colloidal mixture, creating an efficient optical sieve. This technique has rich potential for analysis, diagnostics, and enrichment and sorting of microscopic entities. We further show that particles can be permanently immobilized on the photonic crystal via photopolymerization of the trapping medium. ❧ Optical assembly has potential applications in bottom up fabrication of nanostructured materials. The ability to dynamically manipulate the configuration of optical assemblies can lead to reconfigurable photonic matter with variable optical properties. We present a photonic-crystal design which supports multiple guided-resonance modes in a narrow spectral range. Introduction of mutually-orthogonal slots within a conventional lattice allows us to create polarization-sensitive guided modes with distinct near-field periodicities and tunable resonance wavelengths. The device can potentially be used as a reconfigurable optical trap, multiband tunable filter. ❧ We had also explored the macroscopic applications of optical forces in photonic crystals. The Breakthrough Starshot initiative has proposed to use laser radiation pressure to propel a lightsail to an exoplanet. One major challenge is the effect of laser beam distortion on sail stability. We propose and investigate the use of lightsails based on Kerr nonlinear photonic crystals as a passive method for increasing sail stability. The key concept is to flatten the dependence of reflected power on incident power at the laser wavelength, using a specially designed, guided-resonance mode of the nonlinear photonic crystal. We use coupled-mode theory to analyze the resonance characteristics that yield the flattest curve. We then design a silicon nitride photonic crystal that supports a resonance with the desired properties. We show that our design simultaneously provides both high stability and high thrust on the sail, unlike designs based on linear materials, or differential sensor. ❧ In the second part of the dissertation, the surface plasmon resonance in metallic nanostructures is explored for modifying the thermal emission in engineered nanostructures. Nanophotonic structures can be used to break the fundamental constrains on conventional thermal emitters and realize narrow spectral response. We present a switchable system with narrowband emission using metallic gratings and phase change materials. The phase change material turns a surface plasmon mode on or off by its insulator to metal phase transition. We proposed two design architectures and analyze the limits on the emission switching within the frame work of coupled mode theory.
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University of Southern California Dissertations and Theses
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Asset Metadata
Creator
Krishnan, Aravind
(author)
Core Title
Resonant light-matter interactions in nanophotonic structures: for manipulating optical forces and thermal emission
School
Viterbi School of Engineering
Degree
Doctor of Philosophy
Degree Program
Electrical Engineering
Publication Date
12/09/2019
Defense Date
10/15/2019
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
absorption switching,coupled mode theory,emission control,guided resonance,hermophoresis,laser sail,laser sail stability,nanophotonics,narrow-band emission,nearfield optical manipulation,OAI-PMH Harvest,optical force,optical reconfiguration,optical sorting,optical trapping,optical tweezers,optothermophoretic trapping,permanent optical assembly,phase change materials,photonic crystal,photo-polymerization,radiation pressure,selective optical trapping,slot photonic crystals,surface plasmon resonance,switchable emission,tunable filter, dual-band filter
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Povinelli, Michelle (
committee chair
), Cronin, Stephen (
committee member
), Ravichandran, Jayakanth (
committee member
)
Creator Email
aravindk@usc.edu,aravindkrishnankaithacode@gmail.com
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-c89-247849
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UC11673209
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University of Southern California Dissertations and Theses
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The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...
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Tags
absorption switching
coupled mode theory
emission control
guided resonance
hermophoresis
laser sail
laser sail stability
nanophotonics
narrow-band emission
nearfield optical manipulation
optical force
optical reconfiguration
optical sorting
optical trapping
optical tweezers
optothermophoretic trapping
permanent optical assembly
phase change materials
photonic crystal
photo-polymerization
radiation pressure
selective optical trapping
slot photonic crystals
surface plasmon resonance
switchable emission
tunable filter, dual-band filter